I do understand what you're saying here but consider this: ALL notebooks will heat up when placed on a soft surface, by their very nature they rely on having at least some circulation under the unit. On a MBP, and presumably a MBA, the aluminum case helps to dissipate the excess heat. On a plastic laptop that doesn't happen but that heat is still there, the difference is it's the internals that take the brunt of it. Speaking as someone who had a 15" MBP before exchanging for a 17" it does take some getting used to but in all honesty I now prefer the metal case to a plastic one as at least you know when it's getting a tad toasty.
Oh, and just for the record, we have had something like 200 Dell laptops through our department in work over the last three years and every single one I've had the misfortune to work on has gotten warm on the wrist rests even while sitting on a desk if they're working hard. Same with the handful of HP's and even Sony machines.
