Logic 8 and Logic Pro 8/16 are higher performance than their predecessors (Original Logic and Logic16) and therefore consume more power. This power has to go somewhere, so it is dissipated as heat.
For Logic Pro 8/16, when the device is not actively sampling, the current consumption is significantly reduced to the point where the device should remain or return to room temperature between captures. These devices should only get warm after a long capture.
Logic 8 does not feature the same power saving mechanism as found in Logic Pro 8/16, and therefore, will stay warm to the touch as long as it's connected to the PC.