Pentium M (Centrino), Apple cpus (made by Motorola, etc) and Athlon 64 cpus all run at drastically lower mhz and thus generate less heat than Pentium 4M's.
The Pentium 4M chips are at or above 3mhz but the Pentium M (Centrinos) are at 2.0 mhz or lower and the Athlon 64's are also in the 2mhz range or lower depending on chip speed.
You can tell which cpu is the most energy efficient (longer battery life and less heat means thinner notebook cases) by looking at the notebook thickness and weight.
The Pentium M (Centrino) has the least heat and longest battery life on the PC side. Apple is excellent also in its own realm.
A Pentium M Centrino notebook gets 3.5-5 hours or more battery life in some cases (depending on whether wi-fi is turned on or not, etc) while the Athlon 64 notebooks get higher battery life then Pentium 4M's but less than Pentium M's (2.5- 3 hours I think but not exactly sure).
At any rate, you can easily compare Athlon 64 notebooks to Pentium M notebooks with the same LCD size and see that the Pentium M's are thinner and lighter than the Athlon 64's (see hpshopping.com for example).
For example the HPZT3000 15.4" Centrino is 6.5 pounds -- I believe the HP and Compaq Athlon 64's (same LCD size) are about 7.8 pounds and they are thicker (as more space to cool the notebook is required).
Both notebooks are excellent -- the Athlon 64 has the future benefit of 64bit but the Pentium M gives the best performance/battery life/weight ratio currently with 32bit.
The HPZT3000/Compaq X1000 series fan does not even run at all when using wi-fi or Microsoft office etc (only comes on at startup, when using DVD or CD burner or using hard drive extensively (games). Thus, it so efficient the fan is not even on and the notebook is silent much of the time.