Apple has released an update to iTunes, bringing the latest version of the program to 9.1.1. This update addresses a few stability issues with VoiceOver, a problem with VoiceOver and Genius Mixes, AAC file conversion problems when synchronizing, and overall performance and stability (Updated: 4/28/2010, 8:50am).
The update is available via Software Update, and weighs in at 102.1MB. If you have not already updated to version 9.1, you do not need up update to that version before applying this update.
The update is also available for direct download from Apple's support site: http://support.apple.com/kb/DL1036
UPDATE: If you use the downsampling option for your iPod, which will convert all transferred music to 128kbps for the transfer so you can fit more songs on your iPod, your first synchronization after this update may download and resample all your previously transferred music again, which can take a long time. Additionally, iTunes may reject some songs in this process. If this happens then try re-importing them from your source (CD or otherwise) to see if the songs will transfer afterwards (we thank MacFixIt reader "John" for bringing this to our attention).
This re-download should only happen once, and is probably happening because Apple addressed some issues with this process in the iTunes 9.1.1 update.