I believe most are powered by USB from the notebook/netbook. And the main concern is not cooling, but battery usage. If you have one of the large notebooks that do generate a lot of heat, then the battery life REALLY goes to pot.
Win7 64 bit
Intel i7 920 watercooled
Asus P6X580 Premium mobo,
GTX280 GPU watercooled
Synology NAS 207 Network Server with 2 Hitachi Terabite drives
LG NAS N2R1 with 2 ea 2 tb drives
LINKSYS PLK300 Powerline Network Kit