WordPress 2.9 presented a few bugs which occured in certain contexts. Theses bugs have been fixed in a WordPress 2.9.1 RC 1 version. RC versions (Release Candidate) do not appear in your dashboard. However, you can update via Tools ->Upgrade. If you still notice a strange behaviour, please report it in the WordPress forum so the team can fix it. The definitive WordPress 2.9.1 version will show in your Dashboard as ready to upgrade within a few days.