I would love to be able to do this as well. But I don't think this is really practical for MacBook Pro, because the palm rest is alumnium, which helps transferring the heat from processor, hard drive or optical drive to the LCD/LED Back-lit display, which may or may not cause potential damage. I would NOT try to do it until Apple supports it.
Even with AC on, I some times need to keep a fan blowing the keypad and palm rest area while I am building my projects with XCode, takes roughly 15 minutes to build from a location private from spotlight. Otherwise, putting the palm on top of the hard drive or keypad is like slowly cooking yourself. Maybe that's why Apple claims this is a portable, but not a laptop.
I am not sure if any Apple engineer is monitoring this topic. I would really like to submit this as a feature request for the OS. I know you guys have done a good job on eliminating the heat issue in the 3,1 generation compare to the first. But I still don't feel safe for closing the lid while the machine is running. If you want an example, take a look at IBM Thinkpad T series. I really feel more comfortable using my T40 instead of this MBP I got. But I still buy the MBP instead of Thinkpad, and try to get used to the problems, because I want to be able to run both Windows and Mac.
MacBookPro3,1 (Intel Core 2 Duo 2.2Ghz, 4GB memory) Mac OS X (10.4.10) iMac4,1 (Intel Core Duo 2Ghz, 2GB memory)