UPDATED: Apple has some brief notes on what's new:

iTunes 8.1.1 adds support for renting HD movies and provides a number of bug fixes, including addressing issues with VoiceOver and syncing with iPhone or iPod touch.

iTunes 8.1, released last month, has just been given a minor point release update to 8.1.1, likely to provide what Apple verbosely refers to as "bug fixes, performance enhancements, and improved compatibility". Hit Software Update if you gotta have it now, now, now, or wait and see what the early installers have to say before you leap.