check this link. should help you to find other posts if you need to. the idea is to vibrate the key with a palm sander or other such device while you turn it. Then you must remove the lock cylinder ring and replace the ignition assembly or you'll end up doing this again.
http://www.peachparts.com/shopforum/showthread.php?t=122926
I've never done it, but it makes sense. I've sat in my car trying for a half hour until it finally unstuck. Had to leave the key in the lock for a few days while I waited for the new part to arrive. Luckily, I work on a Navy base, and there wasn't much chance of it being stolen.