2.5" HD's tend to run cooler for a number of reasons, the first being they are really designed for laptops, hence you need a device that runs cooler so as not to generate too much heat and thus cause any number of heat related problems. The design is such that the spindle motor runs at a slower rpm and I would imagine the rest of the electronics is also designed with power consumption in mind, thus less power draw means less heat once again. This is the reason they tend to cost more then there desktop counterparts.
What I'm waiting for is IBM's microdrives to get up to about 20GB or so. These would be killer for any kind of portable machine or inside the cramped quarters of an A600