If it seems odd to mention heat production from servers then you have not operated a server farm.  Servers generate a substantial amount of heat due to their switched mode power supplies. Adding to the problem, servers are concentrated in a compact amount of space.  It is mandatory to move lots of heat away from the servers to keep the equipment effectively cooled down.

Keeping data centers cool is important because overheated computers can perform poorly, lose data or crash. Unfortunately, electricity is on the rise in US and the recent record oil prices are sending shockwaves around the world in certain industries. Global demand for oil has never been higher, lifted by heavy consumption in the US and the fast-growing China.    

The Xeon is more reliant on frequency-growth and expensive (up 4MB of L3 for the Xeon MP) cache memory to increase the performance as opposed to the Opteron which is focused more on increasing the instructions per clock. The Opteron has no L3 cache and instead has a more generous L1 and L2 cache of 128KB and 1MB respectively.  And unlike the Xeon the front-side bus (memory to CPU interface) runs at the same speed of the processor.  The point is that higher frequency means higher heat output. 
 

There is also the Athlon64's Cool and Quiet feature, which automatically reduces the Athlon64's clock rate when the processor runs idle. This function thus slashes power drastically, which is similar to Intel's Speedstep and AMD's PowerNow! for mobile processors. However, the  motherboard makers are so far not being aggressive to implement this function which is too bad because energy is becoming a bigger issue by the day.  No product from Intel has been able to supply an answer to this either.