Sounds like the little gear that the starter engauges to crank the engine isn't clicking into place. I can't remember what it is, an actuator of some sort, that when you hit the power to the starter gets lined up to the engine to turn it over.
If you just hear a really fast "wrrrr" noise and no pop from the engine, this is most likely your problem. And it's a safe bet to say you'll need a new starter.
Kevin (President TBEA)
"WAAAAAAARRRRRIORS, come out to plaaaaay."