The spy satellites that's coming down in a few weeks has been labeled a hazard, and will be destroyed using a Navy missile sometime soon. It's not just that if the satellite is allowed to come in on its own, large pieces could rain down on people. The bigger concern by the government, they say, is the hydrazine fuel inside. This is toxic, and if the satellite is destroyed while still in space, the fuel will disperse and be destroyed when it reenters the Earth's atmosphere.
But what is hydrazine, anyway, and why use it if it's toxic? Astroprof has written up a very detailed yet readable blog post about the stuff. I learned quite a bit about hydrazine from reading that.
Oh, and that crack above about the bigger concern of the government? That's because this is a spy sat, and I don't think it's too suspicious to wonder out loud if their real biggest concern is secret technology falling onto foreign soil.
But to be sure, the hydrazine is a real concern, so blowing the thing up is the right decision anyway.
Also, Heavens-Above.com has a page on the satellite, with a diagram of its orbital height (it drops fast) and another showing its ground track and current position. Very cool.