Upgrading my wordpress automatically was no problem until yesterday
Since minutes I can only see this message on screen:
…
briefly unavailable for scheduled maintenance. Check back in a minute …
thanks to http://www.wendycholbi.com/2011/05/wordpress-upgrade-fail/
Simply access your site with FTP (filezilla) and remove the .maintenance file in the root wordpress directory.
In the browser, now just enter the URL of your sites’ homepage (not the core upgrade php page!) and the problem should dissapear.