Heh, it's called Visor, from the makers of Quicksilver:

http://docs.blacktree.com/visor/visor

You can hack Terminal.app so it becomes a background app too, like Quicksilver/ActionGear:

http://www.macosxhints.com/article.php?story=200107011915182...

I also made it a "login item" to start on system start up. System prefs -> Accounts -> Login Items.

So now, I always have a command line prompt handy =P.