I'm not a helmet expert by any means, but here's my understanding of the differences. Skate helmets are not certified for impact ratings and bike helmets are certified. That is, skate helmets, while better than nothing, may not offer as much protection as a bike helmet. However, skate helmets that aren't certified are usually a lot more comfortable as they have soft foam liners. Bike helmets usually have harder foam liners, which means they can absorb a bigger impact, but the foam doesn't rebound. That means that you should replace the helmet after any significant impact. Skate helmets will rebound because the foam is softer and squishier, but they can't protect your head against harder impacts.
My personal feeling is that, for my kid, skate helmets are fine until he starts doing really big stuff. Getting kids to wear helmets is hard enough, so compromising a little bit on protection for comfort is worth it to me. Now, as soon as he starts going bigger stuff, like over 10-foot airs, we're going to have a discussion about helmets. If he starts doing really big stuff, he'll be wearing a motocross helmet. I don't care how uncomfortable or uncool it is, he's going to have a functional brain at 30. :)