I'm not sure it would be half the heat. It probably is just a bit warmer, and just a bit more power. I was reading some reviews at Silent PC Review yesterday and they measured some 7200RPM drives to only take up 2W more under seeks than 5400RPM drives, and 0.2W more idling.
Well if you compare the latest single platter 7200rpm against the oldest 5400rpm of 6 years ago, then yes perhaps it won't differ all that much. But generally you could say that 7200rpm consumes twice the power of 5400rpm disks.
Idle power consumption comparison:
Intel X25-M with DIPM: 0,075W
Intel X25-M without DIPM: 0,5W
2,5" notebook drive: 0,7-0,8W
2,5" Velociraptor drive: 4,5W
3,5" 5400rpm green drive: 3,5-4,5W
3,5" 7200rpm single platter: 5-6W
3,5" 7200rpm multiplatter: 7-9W
Any other power consumption measurement (for example when seeking) is irrelevant; unless you do nothing but seek all day long. The total power consumption of HDDs do not exceed the idle power consumption much. Due to their mechanical nature they always consume power due to the air-friction of the platter rotating; the added power consumption of seeking is negliceable unless you do laboratory tests and let them seek all day long.
A HDD doing nothing and a HDD reading at 150MB/s differ only slightly in power consumption; heavy seeking and doing 0,1MB/s requires the most power consumption.
As air resistance increases exponentially, a 7200rpm will be only 25% faster but use 50% more power; if all the rest of those two drives remain unchanged. The 2,5" Velociraptor at 10.000rpm uses more than 4W of power; which is ALOT for such a tiny drive and the reason it needs excellent cooling in its 3,5" caddy. Though even with all this 'brute force' a simple 5400rpm green drive could beat the Velociraptor; in fact most green drives do at this moment; thought the latest 600GB Velociraptor is still a tiny bit faster.
Concluding, i think 7200rpm will become much less popular than they currently are. HDDs can't do IOps very well so we won't use them for that in the future. All we would use them is mass-storage for sequential workloads. 5400rpm are almost as fast as 7200rpm in that area, so would be more logical. SSDs would be used for all random accesses instead.
At this moment, if you do not have an SSD, you can still consider the 7200rpm version. Otherwise i would pick the 5400rpm and benefit from potential higher reliability and endurance, sound production, vibrations, power consumption and heat generation.