It's come up a few times and from what I can tell the basic answer is, meh. Even wikipedia's entry seems pretty ambivalent about their use. There are a few problems. It's almost impossible to build a helmet thats going to provide serious protection while still allowing enough ventilation so you don't overheat your noggin. Another problem is the testing method which makes it so the only way a helmet will pass is if the foam is very hard, which means that your head deforms before the foam. If the foam doesn't deform then it defeats the point of a helmet. From what I read it seems like a helmet is quite good for preventing scrapes and even serious skin damage. The problem is they don't seem to be effective at preventing life threatening injuries. Plus they actually increase the risk of rotational injuries. You don't want your brain to twist inside of your head, its very bad.
To be honest though I'm biased. I hate wearing a helmet, it's uncomfortable and goofy looking. I do my best to ride safely which I think provides far far more security than any lil chunk of foam and plastic can provide.