There is a bug in WordPress 2.9 that may cause scheduled posts to not publish at the set time. And if you try to then publish the post yourself, it won't work. I ran into this this morning with my eBay + Kijiji + San Jose = Pure Awesomeness post. The problem is wp-cron. The cron file controls scheduled jobs. If it does not run, then the jobs don't run.
The fix can be found on the Semiologic site. There are actually fixes for 3 different problems on that page. I installed them all just in case.