Bad idea! These things are marginal on cooling, and running it in "clamshell mode" is almost certain to fry something (or cause the video chip to come away from the motherboard as is common on these models).
If you have to, however, take a look at Extended Desktop (http://www.rutemoeller.com/mp/ibook/ibook_e.html) which enables operation in clamshell mode.
Once again, I advise against it. Doing so for extended periods of time is almost guaranteeing yourself a hardware failure.
That PowerOn guy has a TON of these iBooks. I bought a similar iBook from him (800 MHz 12" G4) in early December. I absolutely love it, but it is just a little slow. I want something compact that can play videos hosted on the network back at full screen without ever dropping frames. My PowerBook G4 1.33 GHz and 1.67 GHz models can do this fine, but the iBook G4 800 can't. I'm thinking the 800 MHz is just on the hairy edge...
Peace,
Drew