AMD have termal issues in the top of line Opterons etc.
High-end stuff tends to be MORE efficient because they have to run 24/7.
The 2xx/8xx Opterons are actually thermally cooler than the FX line.
Regardless, when Woodcrest / Conroe comes out you'll all toss your crappy P4s and pray to the Core gods for forgiveness for ever believing the Netburst architecture was even worth owning...