I serendipitously discovered this after partially botching the command-N shortcut to open a new shell window in Terminal.app recently: Command-Option-N will open a new shell without loading the settings from your stored window settings (dimensions, background and text colors, cursor style, etc), using the default 80x24, black on white settings instead.
[robg adds: This is actually documented, sort of, but hidden. Hold down Option before selecting the File menu, and you'll see that New Shell has become New Factory Default Shell. However, as Terminal lacks the "live updating" of menu items seen in many other apps (ie Finder), I hadn't discovered this one yet. My usual routine is to activate the menu, then press and hold the various keyboard combinations to see if any of the options change. In Terminal, none of them change, but that's because the menus don't update in real time. You have to hold Option first to see this command -- which, as best I can tell, is the only such modified command available in Terminal's menus.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007080811340314