The 15" pbook gets so hot it is uncomfortable to have it on your lap, and that's only from running Safari and Mail. I've researched this pretty well, and what you need is a flat surface to insulate your legs from the heat AND/OR draw the heat away from the powerbook to keep it cool.
First I bought the Lapdesk from Targus. Does NOT work very well at all. The Lapdesk still gets too hot under my legs even though it's made with plastic, and I doubt the powerbook gets any cooler.
So I ordered the iLap after considering the other options. The iLap is made of aluminum which effectively increases the heat loss from the powerbook. The bottom of the iLap still gets warm, but certainly not as warm as the powerbook without the iLap. I've also been monitoring the pbook's outside temperature manually (since, as someone pointed out, this OS does not refresh the internal temperature sensors), and it does feel cooler than it did when I had it lying on my lap. Best of all, because the bottom of the iLap is suspended about an inch above your legs, you get pretty comfortable, too. I'm keeping this one, at least for now. I've had it for two weeks and I'm pretty happy with it.
The only downside to the iLap is that it's not exactly portable since it gets pretty bulky to just put in your backpack or whatever carrying case you have. I think there are other options, with active cooling (fans) that might work, too. When I get unhappy with the iLap, I'll probably try the Targus chillmat.
