In fact, some crowds are just mobs – no wiser than individuals, and frequently less so. (A favorite columnist in Anchorage, Mike Doogan, used to say that talk radio is "a way for people to get together and pool their ignorance.") To discover wisdom, it's essential to assemble a genuinely diverse crowd, and create systems that make sure everybody gets listened to and considered.
Jay Rosen posts a good interview with Surowiecki at PressThink, here. You'll enjoy the interview with Emily Gordon, I think. Here's a taste:
... one of the consequences of the digital divide is exactly what you suggest: the “crowd” of people online is not as diverse as it could be, and therefore is probably less collectively smart than it might be, since we know that cognitive diversity is fundamental to collective wisdom.