This has happened to me a few times now when theming. Does not seem specific to any one theme so I believe it is a Drupal core issue. When theming and having certain regions disabled I cannot get the log in page to come up. Even after trying all the url tricks such as /user, /?q=user etc. Even after renaming theme folders so Drupal tries loading another default theme. I have read all the threads of people having this problem and there have been times when none of the solutions have worked. The one and only thing that works when that happens is to edit MySQL directly to change the default theme. If you cannot access MySQL directly for whatever reason then you are truly royally screwed.
To summarize, it is possible to be completely logged out and locked out of Drupal with no way back in other than changing the default theme in MySQL directly (and only if you have admin access to the MySQL database) which should never be necessary under any circumstances.
There should be some fail safe mechanism that does not involve editing MySQL directly to change the theme when there is no way to do it in the browser.