If you read the more factual news reports instead of the ones that like to sensationalise things -
It isn't a mosque and it isn't at Ground Zero - this is all propaganda hype that's blown up out of proportion.
It's a community centre to try to build relationships between Christians and Muslims up, so that things like 9/11 don't happen again. And, it's about two blocks away from Ground Zero.
http://en.wikipedia.org/wiki/Park51