Try hitting the re-install now button in the updates panel (after you make a backup of everything).
I am on DreamHost, and had this issue. Via FTP, I replaced out the single file ‘/wp-admin/includes/post.php’ with a fresh copy of it from just the download you can get here on www.ads-software.com. After I did that I was able to access the admin again, from there I went to the update panel in the admin and just hit the ‘re-install now’ button (to make sure everything got updated this time), and everything seems to be just fine now. Just in case this helps somebody who ran into that.
Otto’s comments clued me in to it being an incomplete upgrade, and from there it was pretty easy to fix.
