theme_key does not switch theme when URL user//edit is called
Here is the answer : http://drupal.org/node/1130158
To quote:

user/%/edit is an admin path. See function user_admin_paths for details.
So if you enabled an &quot;Administration theme&quot; at &quot;/admin/appearance&quot;, this theme will be applied instead of any theme configured using ThemeKey.

Also: Don't forget to specify &quot;system&quot; in your settings for theme_key.