It's a good thing I know HTML because after I upgraded to 3.1 on one of my sites, Page and Post only show HTML on both Visual and HTML tabs.
I don't know why this happening on just *one* site (I upgraded three of five), but it wasn't happening before. They are all using the same Atahualpa theme. I haven't upgraded my main site.
Have you tried:
- deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
- switching to the Twenty Ten theme to rule out any theme-specific problems.
- resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
- re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress.