Actually using 2.9.2 now, but not an option.
Earlier today, one of my pages stopped allowing the Visual editor to be used. It is still fine on all the rest of the pages and since all pages are basically variants of each other, I don't think it's a matter of coding. I went looking for other people who have problems and while it seems to be rather common, it seems to be that either the editor works or it doesn't, it isn't page-specific.
I upgraded to 2.9.2 and that didn't solve the problem. I have disabled all plugins, that didn't help. I switched to the default theme, that didn't fix it. I cleared cookies and cache. Nothing. The "disable visual editor" isn't checked. I upgraded my theme to the most recent version. No go. I modified the wp-config.php as suggested and that didn't do it either. I installed a new editor (Dean's FCKEditor) and that didn't work on that particular page. It's only this one particular page, all the others work.