If you ever get stuck in maintenance mode while you were trying to upgrade a plug-in in WordPress 3.0.2, go to the root of your WordPress website and delete:
file from that directory and after that the website should be back and operational.
I was trying to upgrade “Fast Secure Contact Form” plugin and it got stock on it and I could not even go into the administration panel. I was continuously getting the following error message:
“Briefly unavailable for scheduled maintenance. Check back in a minute.”
Now, how do you get the root directory of your website? You can login to your web hosting company (I use 1and1, they have an admin panel) or find a ftp client and call your hosting company. Of course assuming that you have information from your web hosting company.