This bug has been reported
It appears that Apple has implemented 'sandboxing' for the 10.6.3 iTunes update. This means that external apps are unable to use AppleScript to control apps, which is how calibre communicates with iTunes.
I will keep looking at this, as it's possible that there's something else happening. In the meantime, you can either downgrade to a previous version of iTunes, or use calibre's content server.