Morgan Spurlock ate three meals a day at McDonalds for a month, and answered yes whenever he was asked if he wanted to super-size his meal. His reasons were twofold. 1) To make a point about how bad fast food is for you and how evil McDonalds is for trying to sell it to you, and 2) To create a documentary about his experiences. I have not seen the documentary (which is why this is in the Et. Cetera section rather than Video), but I have read a good deal about it including an article by Spurlock himself, in the latest (June '04) issue of Men’s Health, in which he summarizes his story and conclusions (no link, sorry).
In the course of this exercise Spurlock occasionally consumed as much as 5000 calories a day (twice what he would need to maintain his weight), often eating until he was sick. He comes out the other end of the month fat and unhealthy beyond all reason.

A sample of a daily menu:

Breakfast:

Egg McMuffin
Hash Browns
Coffee
Orange Juice

Lunch:

Big Mac
Large Fries
Large Coke
Chocolate Shake

Dinner:

Two Cheeseburgers
Medium Fries
Medium Sprite

Total (per Men's Health): 3833 calories and an ungodly amount of salt, fat and cholesterol.

Now, I am between 5'9 and 5'10, and between 170 and 175 pounds. I don't think I could force myself to shovel that quantity of food down my throat. That is beyond extreme. If you eat that much food everyday you likely have serious psychological issues beyond just habitual overeating. Blaming this on McDonalds (as a proxy for the fast food industry) is rather disingenuous.

An alternative, more reasonable three meals a day at McDonalds might look something like this:

Breakfast:

Egg McMuffin
Hash Browns
Black Coffee

Lunch:

Fish Filet
Medium Fries
Diet Coke (medium)
Vanilla ice cream cone

Dinner:

Chicken McGrill
Medium Fries
Diet Coke (medium)

Totals (source):

2090 calories

88% of the daily recommendation of saturated fat
122% of the daily recommendation of cholesterol (mostly due to the Egg McMuffin)

141% of the daily recommendation of sodium
Still not particularly healthy, but even a mildly active person my size wouldn’t gain weight or be deathly ill by the end of a month. And yes, because of the sodium, you wouldn't want to eat this if you had high blood pressure; and you’ll want to swap out the Egg McMuffin for something lower in cholesterol now and then; but throw in a multi-vitamin each morning and you’re probably OK for a 30-day period.
But even with this more reasonable menu, the fact is that nobody eats three meals a day at McDonalds. Spurlock would no doubt agree that nobody actually does this; he's just trying to make a point. Unfortunately his eating binge doesn't make it. Taking an activity, any activity, to an unhealthy extreme doesn't make a point about the bad effects the activity; all you do is make a point about the bad effects of taking things to an unhealthy extreme.
I eat a good deal of fast food, probably more than I should, and I probably eat at McDonalds twice a week, just for convenience sake (it's just down the corner from work). My cholesterol is just fine. I actually have low blood pressure and, nobody would accuse me of being overweight. I’m glad for McDonalds. I benefit from the convenience. I just don't shovel the stuff down my throat like that, and when asked if I wanted my meal super sized I’d say, "No thanks."
Which gets to the next point. When Spurlock visited McDonalds one of his rules was that if he was asked whether he wanted to Super Size his meal, he had to say yes:

It's not really the burgers and fries I'm worried about, it's the amounts they push, and the amounts we eat.
The implication is that the suggestive sell is what causes people to eat more. It’s the old corporate mind control argument. The supposition is that through the effective use of advertising and promotion and suggestive selling, you are connived into doing something you really don't want to do.

Fast-food chains say their offerings are "sometime" foods and that they're "part of a balanced diet"…You never hear one of these "restaurants" tell you how often you should eat their food…

Oh dear. Are we all such weak minded gluttons that we need to be told how often it is safe to eat fast food? This argument has always smacked of a certain arrogance and condescension, as if the writer is wise enough to see through something that the average schmoe is too dull to comprehend. I could accept the argument that the average consumer might fall for it once or twice, but super-sizing once or twice before you catch on isn’t going to make you obese. You have to keep doing all the time.
Instead of fretting about suggestive selling and advertising, I suppose you could make the argument that the widespread increase in portion size across the board is the culprit. That assumes a person who orders French fries will eat the entire order no matter the size, rather than just the amount that satiates hunger. But if that were the case, as soon as you saw your gut drooping over your belt you would think you'd know enough to stop. In no McDonalds that I know of is it a requirement that you eat every morsel of food on your tray.

Doubtless there is a correlation between increasing food portions and increasing waistlines. But correlation is not causality. Portion size has been increasing for many, many years. I remember when there was no such thing as a quarter-pounder, never mind super-sizing. McDonalds is just a corporation, which is to say they will follow where the market takes them to get the profits. My memory may be faulty, but I seem to remember McDonalds starting the quarter-pounder in response to the Whopper from Burger King and the gigantic burgers from a new kid on the block called Wendy’s. Similarly their recent abandoning of super-sizing and the addition of salads to the menu is following on the heels of Subway’s very successful health oriented ad campaign featuring Jared (despite the fact that Spurlock claims credit for this). It is by no means clear that increasing portion size came first and appetites followed. This has been a very long term process and it's more likely that McDonalds followed the market -- that is their business after all. This is another key point: Everyone knows overeating is unhealthy yet they have continued to do it, despite the fact that alternative choices are available. To say it continues because McDonalds makes it convenient is, once again, to suggest that people are not in control of their decisions.

Here's a possibility that I have yet to see investigated. The average adult weight has actually been increasing steadily for decades -- I have read studies that seem to indicate things started getting bad in the '70s. That probably coincides with the gradual increase in portion size. But it also coincides with the long decline of cigarette smoking. Is it not possible that by abandoning the appetite suppressant of cigarettes we have left ourselves more susceptible to overeating? I have no data, it's just a thought. Perhaps Spurlock will do a sequel wherein he takes up smoking and then sees if he resist the hypnotic suggestive selling by the greasy teenager behind the counter.

It's a worn out old pose to take. Find some societal ill and label the supplier as evil. It can be fast food, violent video games, Spuds Mackenzie, SUV manufacturers, you name it; you'll be very popular. But supply invariably finds a way to meet demand -- cases in point: prohibition, the war on drugs, pirated MP3s. And while it's unlikely that super-sized meals would appear on the black market, it is entirely probable that people would just order twice as much of the smaller portions, say two cheeseburgers and fries instead of a quarter-pounder and fries, or maybe just hit the candy machine at work more often.

That is where Spurlock's thesis falls down. The only real way to fight overeating -- or any other self-destructive behavior -- is on the demand side. By blaming the fast food industry and providing a convenient excuse for the face-stuffers, Spurlock undermines the only possible solution: Personal responsibility.