As you used Software Update to install the most recent iTunes version, I suppose you installed iTunes 11.0.3. This update was released yesterday, and it looks like it was damaged, so download and install iTunes 11.0.3 again from the Apple site > http://support.apple.com/kb/DL1614
To anyone else who may have experienced this problem (I doubt that I'd be the only one):
Over the past couple of days I have left my Macbook sitting idle for extended periods, put it to sleep, & also shut it down.
This morning I tried to reinstall the same download that didn't work a couple of days ago & it has been recognised, installed & seems to run just fine - playing right now I'm happy to say!
I suggest you try doing the same: ie. leave your machine sitting idle till it sleeps & shut it down, before you try the reinstall.