Media upload not working

Have you tried:

– checking through Troubleshooting WordPress 3.1 – Master List

– 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.

– increasing the memory available to PHP:
http://wordpress.org/support/topic/253495#post-1017842