Upgrading WordPress to 3.2 caused the dreaded “Briefly unavailable for scheduled maintenance. Check back in a minute.” to appear.  I think it was related to an incompatible plug-in.  That’s strange seeing as the plug-in in question was disabled at the time.
In short, deleting the .maintenance file in wordpress’s home directory fixed the problem.
See How to fix “Briefly unavailable for scheduled maintenance. Check back in a minute.”