I've recently upgraded from 2.5 to 2.8, and like many others, my scheduled posts no longer are published. NO ONE seems to have an answer to this very common problem, at least I've found no solution. I've tried the advice of many posters on WP forums as well as those found on the web elsewhere.
However, I might have a clue which might help. Background: I had my website (www.tamiasoutside.com) hosted with Laughing Squid when I upgraded to 2.8. Scheduling posts worked just fine for a week or more after that upgrade. Then I signed up for Squid's newish "cloud" hosting, which required I move my site to a new server (and subsequent DNS change). Suddenly scheduling posts doesn't work. I get the dreaded "Missed Schedule" alert in the Dashboard display. This suggests to me that the problem is based with the server (I'll be on to Squid on Monday when they open business).
Perhaps this will help people who are far better skilled than I to find an answer to this vexing problem which is causing a lot of us WP users serious inconvenience.