I'm having this problem too. iTunes isn't playing any tracks. It just won't shut down without INSTANTLY relaunching.
It isn't the audio jack in my case (removing it made no difference).
I thought it was an issue with SRS iWow, as iTunes had recently opening with a dialogue window about having trouble loading a menu item related to this. Uninstalled SRS iWow. No change.
Uninstalled Last.fm - no change.
The problem is so constant and extreme that force quitting only shuts iTunes down for a fraction of a second. As soon as it vanishes from the list, it returns.
I can't shut down, restart, update iTunes or update the OS. Every time I try, iTunes cancells the shutdown (or, in the case of updating iTunes, I can't update until I quit iTunes, which I can't).
The computer is basically crippled. I can't do anything that iTunes doesn't like.
I'm using a 2.66 Cor 2 Duo Mac Mini, 4 GB of RAM and running os x 10.6.8 with iTunes 10.4 (80).
Unless I can make iTunes shut down, these look to be the last versions of anything I'll ever have on this machine!
Help. Anyone, please, help!