Ooooh, this is interesting. I was talking about that with my colleagues just a few hours ago.
Let me explain. I work some evenings at a supermarket, fitting it around uni, and part of my job includes taking all the waste stock (stuff that's gone out of date or is damaged or whatever) out to the bins at the back of the store to be collected the next day. Now, our store is brand new, only opened about two weeks ago, and there are massive walls at the back along with a big gate. However, because the store is built into the side of a hill, people can easily climb over the wall and get to the bins.
Earlier tonight one of my colleagues mentioned that he'd caught a woman with a bag after she climbed over the fence to get to the bins. The bag was full of food, and she claimed to be from some charity giving food to the homeless. She begged to be able to take it, but the guy kept saying no, he wasn't allowed to. Which is true. Thing is, in the UK it's actually illegal for a food retailer to give away food that's past it's sell by date, even if it's free. Reason being that if someone gets ill, the store can be sued, and it's against Health and Safety rules. The silly thing is, most of the stuff we throw away each night is perfectly good to eat still, as long as you eat it within a few days, and a lot of the staff at the store (myself included) agree that we should be able to give the food away instead of throwing it out, because let's face it, no-one likes to waste food.
Anyway, this guy confronting the food-stealer turned away for a split second to see if any of our colleagues were coming to the back of the store for some reason, so he'd have backup, and the woman grabbed the full bag and ran. She had a friend on the other side of the wall who took the bag, and they managed to escape with this food. Well, later on, the guy who confronted this woman kept seeing them out the corner of his eye loitering by the wall, trying to get an opportunity to get back in and steal even more stuff from the bins. Fortunately they didn't make another attempt.
Also of note is that I know some people at university (not well, mind you) who do this sometimes. Though admittedly I know them through the student union common space, the Peanut Gallery, which is generally regarded as the hangout of hippies, liberals, and every student campaign group in Leeds (even though that's a false stereotype, and the Gallery is open to anyone, non-students too). Last year there was even an article in our student paper about such people. There's also an actual name for people who do this: Freegans.
Overall, I do agree with the idea of it, provided you're careful and don't take anything that will actually make you bad. After all, the food is only going to be wasted otherwise, and it's stupid to let all that food just be dumped if it's still good quality (though I can assure people, working in the food retail industry, that not all the food in the bins will be good quality, so I do implore any aspiring Freegans to be careful what they take and to avoid anything that looks dodgy or suspicious, or anything with damaged packaging).
That said, I've never done it myself, nor do I have any desire to. I just make an effort not to waste the food I do buy. So yeah, it's a decent idea, but not something I do myself, I'm afraid...