After upgrading to WordPress 3.1, we found that the visual editor for posts did not show when editing. The tabs show, but the editor function and buttons do not. Instead, a very limited range of buttons shows.
We rolled back and reinstalled, checking all the prerequists. With all the plugins deactivated, the editor worked. So we activated all the plugins and then deactivated them one at a time. When PostRank was deactivated, the problem was cleared.
Whatever PostRank is doing - it conflicts with the visual editor in WordPress 3.1 and no other plugin we're using seems to have the same problem.