There are hacks (do a search for "clamshell mode"), but the only way to do this without changing anything is to connect an external monitor, keyboard, and mouse.
It'll be good if I can keep my pb running when I move around and still need to use the internet. Now I need to leave the lid open a little else I'll be d/c.
I've been wondering about the same thing - I want to use my PB's hard drive while the lid is closed - I've been envisioning a stop motion movie (using iStopMotion) but having the PB open while video recording in motion is a bit tricky.
It tends to be a bad idea to do this, especially when you move around with it. The hard drive will still be spinning, so if you bump/drop/harm it when you move it, then there's always the risk of disk damage. Furthermore, the laptop is designed so that some heat is released from the keyboard; closed-lid operation inhibits dissipation of that heat.