Until last night, I thought the only way to run a Powerbook with the lid closed was the Apple-supported method of plugging in an external display, external keyboard, and external mouse. But now there's an application called
Sleepless ($5 donationware) that actually keeps your computer running when you close the lid. I've tried it for a few minutes at a time; so far, it appears to work as advertised. The display turns off, but the computer does not.
***NOTE: Powerbooks are designed to be able to run with the lid closed, but iBooks are not; you risk overheating and destroying your iBook if you run this progam on it.
[
robg adds: There's a long discussion on using laptops with the lid closed in
this older hint about closed lids and external keyboards/monitors.]