If you had the Diablo III beta installed, you might come across error 33 “battle.net is down for maintenance” while trying to log in. This is because there is an old registry setting pointing at the US rather than EU login server.
Take a look in HKEY_CURRENT_USER\Software\Blizzard Entertainment\Battle.net\D3.
I solved this by deleting the entire ‘Blizzard Entertainment’ key. From orbit, just to be sure. Works like a charm.