macosxhints.com has a hint on how to run your MacBook with its lid closed. They link to this apple technote, but its not just for your MacBook. Powerbook G4 and MacBook Pro owners are just as capable, and can have the joy of their portable beasts plugged into an external monitor as a “faux desktop”. You just have to have an external keyboard or mouse plugged in.
Bluetooth keyboard users get their own technote.

The hint mentions a side effect is a quieter mac.