WordPress 2.9 seems to have a bug which is annoying the hell out of people. The problem is occurring with scheduled posts and missed cron jobs. Some users reported that scheduled posts were either missed or posted in the past.
The main reason why this is happen is because of some code introduced at the last minute, which in turn has apparently broken the WordPress cron. This is only affecting a minor percentage of users.
If you are experiencing a problem with scheduled posts or other Cron jobs not running on your blog, you can find the patched files on this page, along with the description of errors. WordPress might release 2.9.1 in a day or two to fix this issue. In the meantime you can use the files from the page above to fix the missed schedule posts and cron jobs issue with WordPress 2.9.