WordPress has announced the release of WordPress 2.9.1 today. There were a bunch of major fixes and you can read the full list of changes here. Apparently some hosts weren’t able to properly handle the trackbacks and scheduled post changes in the overall 2.9 release.
You should be able to upgrade by selecting the Tools menu on the left side of the admin interface and then selecting Upgrade from the menu. If you have been using the beta release you should upgrade as well.