On my Macbook Pro, I wasn't able to use just the tilde key (~) as an activation key in Visor (a Quake-like terminal). The app's preferences window allows only tilde with some of modifiers like Option or Control. But you can change it back to the single-key Quake-like behavior if you wish. First quit Terminal if its running.
Open ~/Library » Preferences » com.apple.Terminal.plist in Property List Editor (or PListEdit Pro). Find the dictionary entry called VisorHotKey, open it, and make it look like this:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100322063635504