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That wont cool your computer down, as whatever interface you place it on will be an insulator, so both the matt and computer will heat up.

Just get a stand that allows air to flow underneath it.
 
My 17" MBP gets warm, but not hot enough to worry about it (and I use it on my lap rather a lot). I dunno what the 15" models are like, but I'd doubt they get _that_ hot. I'd wait until you get it and see whether you need anything...
 
Stands are really only useful if you have the computer in one spot all the time and your using a wireless keyboard and mouse. You'll probably find most of the time it's not necessary if you just prop the back of the computer up a bit.
 
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