A recent study conducted in London showed that 21% of the 578 people stopped on the street by the researchers where willing to reveal their passwords in exchange for a chocolate bar. The obvious flaw in this study is the fact that the researchers had no way of verifying that the passwords provided were real, but I wonder how many people are devious enough to realize that giving a fake password will still get them that little piece of heaven.
The Incredible Hulk was already in the can when this study was released, so I have to give Zak Penn (or Edward Norton who apparently did an uncredited rewrite of the script) credit for coming up with a similar social engineering technique. Towards the end of the movie Bruce Banner, played by Edward Norton, needs to get into a high security university research building to gain access to a computer network. How does he do it (spoiler)? He brings several pizzas from the pizzeria that he was hiding out in and uses them to bribe a security guard and a graduate student into looking the other way while he accesses the network with his ex-girlfriend's user name and password.
In real life this probably would not have worked on a trained security guard--I recently saw someone try something very similar and fail--but there is no doubt in my mind that the graduate student would have handed over the keys to the kingdom for a free pizza.