WordPress 3.0 Error: Briefly unavailable for scheduled maintenance
I have upgraded this blog to WordPress 3.0 not long after it was available for download. Everything went without any major issues. I only use it without special hacks. One thing that makes this version a little bit different is that WordPress will bring your site offline if there is/are something not working. For example, when you’re upgrading your plugins. WordPress will temporarily bring your site under maintenance mode.
If all process were completed, you should be happy. Otherwise, you will need to do some extra works. WordPress will display this kind of error message: “Briefly unavailable for scheduled maintenance.” You will be locked out from your admin area. Your website will be inaccessible. Solution? Easy. You need to go to login to your webhosting server, and remove .maintenance file. You can find it in your blog folder — the same location with your wp-config.php.

Almost done. But, I think I'd better go home now. Morning meeting, getting ready at 8.30 AM. Howahhhh! Let's see... 
Well I could not find your name anywhere, but thank you very much for posting this post. I really thought that I had destroyed my installation of WordPress and that I was about to loose all my data. Deleting the .maintenance solved all my problems! Thank you again
Hey, another thanks here. Had a mild panic when site freaked out updating Disqus comments. Was thinking rebuild or something horrific. This made it dead easy. Thanks!