WordPress 2.9.1 has been released, and is now available for downloads. This release fixes several minor issues, and the bugs which caused problems with scheduling posts and pingbacks. Upgrade is not really necessary unless you are facing any of the issues listed here.
Another interesting fact shared by Ryan Boren is that WordPress 2.9 has already clocked a million downloads till date, now that is really good. If you have not yet upgraded to WordPress 2.9, read my earlier posts describing some of the best features in WordPress 2.9 or visit this entry on WordPress Codex.
You can download WordPress 2.9.1 from here or use the automatic upgrade feature to upgrade from the admin dashboard.