I'm just waiting to find out how many people end up frying their AppleTV by sticking in larger hard drives that could potentially generate heat beyond the tested tolerances of the ostensibly fanless unit.
When they realize they've voided their warranties and are out $300 plus the cost of the new drive, maybe they'll be thinking, "Hmm, maybe Apple limited it to a 40 gigabyte drive for a good reason."
Somehow I doubt this will happen. Harddrives have never been huge heat producers. And besides, increasing the capacity doesn't increase heat output. The only thing that might is increasing the RPMs, and even then, it's minimal, although, I wouldn't go sticking 10K raptors in there...
Also, i'm sure apple put 40 gig hard drives in there to keep the manufacturing cost down, not because of heating issues.