Looks like more and more people have problems with upgrading to WordPress2.5.*, comments:

Last evening’s project was to upgrade to 2.5 on a subdomain of one of my web sites. So I go through the tired and true procedure:
* Backup the site.
* Backup the db.
* Deactivate all plugins
* Delete all WP files, except wp-content
* Upload the new files
* Run the upgrade script
Then go back to the dashboard. Yep, you guessed it… I’m still at 2.3.3