Lies all lies.....
as many people who have had the sleep function fail on there dual usb ibooks can attest to while the unit does get warm, does doesnt really approach 'hot'. The misconception about the dual usb having keyboard vents for heat is wrong as well, as this was only true with the clamshell ibook, and in addition the ibooks have these nifty loud ass things called fans that tend to kick on when it does get a little too warm.
The main reason the apple document and some more informed people will reccomend not using your ibook while its closed has more to do with mobility and the hard drive.
See some people would like to use the ibook much like a ipod and run with headphones and the lid closed, this created a big problem for the hard drive which was never designed to seek files on the platter while in motion, which can cause damage or even catastrophic falure of the drive due to the heads not being parked while in motion.
but you say the ipod has a harddrive in it, how does it work? well I would guess on top of using a drive that can handle the jolts and vibration (which partially explains ipod costs) the ipod uses the onboard memory to keep the hard drive seeking at a minimum.
so can you run it closed? probably.. its a hardware switch that triggers soft ware to sleep it, and in some cases it doesnt sleep automaticly, but if its on a table not moving you do run the risk of head 'possibly' damaging your LCD, but really, wont your keyboard do more damage? and why dont you have a screen protector in there anyway?
the solution is of course just to put a pencil in there and almost close it, or whatever