This has happened to me twice today...
The dock will have one application (in this case, snak) bounce up and down repeatedly (which I have it set to do if someone is talking in the channel), but I can't switch back into Snak. I also can't use apple+tab to switch applications, and if I go to dock preferences, I just get the spinning rainbow wheel.
The way I fixed it was by force quitting the dock via activity monitor, and all is back to normal. I've only been using OS X for a week, and I am still in the habit of where the XP shortcuts used to be (like I sometimes hit apple+q when I mean to hit apple+tab etc., pretty annoying).
Is this an issue with Snak, or did I somehow hit a key combination that froze the dock or whatever? When this happens, I can't even use the dock to switch applications, and the icon magnification doesn't work.
edit: Possibly relevant...I have quicksilver running, but I know I couldn't have executed any command without seeing its popup thing?