Same problem as everyone else:
My wordpress 3.0 is stuck in maintenance mode, after I tried to upgrade a plugin, the plugin upgrade failed stating:
"Upgrade failed. Could not remove old plugin."
Now the whole site even the /wp-admin just shows this message:
"Briefly unavailable for scheduled maintenance. Check back in a minute."
I deleted .maintenance but it didn't work.
I created plugins.hold and a blank plugins folder.
This worked, so then I started activating plugins one at a time, then refreshing my blog page.
Finally turned out that a whole different plugin could not be activated for some reason or other. Since I hadn't even gotten to upgrading that plugin yet, and it had worked before trying to upgrade the other plugin, I'm stumped as to why it would start interfering now.
But anyway, deleted the offending plugin, manually put the new one in, and now everything is fine.
This has been happening since the last version of WP was released, so it's definitely a problem on their end.
Definitely no more automatic upgrades for me, either, until they get this fixed.