If you Xbox 360 is overheating (while in an open space) then it's a malfunction that has to be fixed by MS. There is absolutely no need for such an accessory if your 360 is working normally, like a vast majority does.
My Xbox360 gets hot as hell. Hotter than any other entertainment electronic device I have ever owned. I have played it for hours on end without an issue, but I cant say when I go to turn it off and even the disk eject button uncomfortably warm, that im not a bit worried.
That said, I would never buy a cooling device for it cause I always buy extended warenties. If it gets to hot and overheats/damages itself, ill just replace it for another.
I just got my 360 two days ago, I've probably played 15 hours so far and it doesn't get hot at all (the power brick only gets mildly warm as well). The fans are quite quiet too, the dvd drive is another story though (MS could have spent a few quid more on the drive I think).
Like someone's already said. If your Xbox is overheating why would you buy something to make it cooler, instead of sending it to Microsoft and getting it fixed for free?
My campaign of terror? The only reason I bash them is because they fucked up royal when dealing with customers including myself. The console has problems you have to admit that, there wouldn't be pictures of overheating GPU's all over the net, People posting stories about recieving 3 red light crashs for no reason (rod), nor horror stories like mine and Argyle (who's still waiting for his HDD to come in I believe) about MS tech support which treats screws up mutliple orders, makes promises they can't keep, and so on.
My console never really overheated, well at least I don't think so. I had an incident that caused my 360 to die which make that screen turn all blue with black stripes. I guess that could have been a overheated GPU/Processor but no way to test that out since it was DEAD. If companies are producing so many cooling options with the 360 in mind then maybe their acknowledging that there is a problem and here's a solution for your consoles.