After a short beta testing WordPress 2.9 “Carmen” is out and available for download or you can use the automatic upgrade (Tools > Upgrade) .
This release is named in honor of the jazz vocalist Carmen McRae.
Some users have reported issues with this version and incompatibilities with some plugins (RSS, Cron and scheduled posts, pings). Is recommended that you do a full backup of your database before attempting the upgrade.
And if you find issues with your WordPress installation after uploading the files of this new version or with the automatic upgrade check this post at Semiologic that provides 3 patched files that solve some of the issues that most users are facing.
I think that WordPress to release version 2.9.1 in a few days to properly address this issues.