Apple has released to fix a number of important bugs. From Apple's perspective, the most serious are probably those that resolve issues browsing the iTunes Store, but users will appreciate fixes for iTunes becoming unresponsive or crashing.
The release notes also claim improved app syncing for the iPhone and iPod touch, a fix for a problem syncing podcasts in playlists to an iPod or iPhone, and proper sorting for albums with multiple discs. Plus, the Genius feature is now automatically updated to show Genius Mixes, and the zoom button should now switch to the Mini Player properly.
Lastly, iTunes 9.0.1 includes a fix for, a maliciously crafted example of which could lead to unexpected application termination or arbitrary code execution. (  is a format for storing playlists.)
The update is available via Software Update or as from Apple's Support Downloads page. It's available for Mac OS X 10.4.11 and later, and the usual versions of Windows.