The myth is that SOI is the reason, but this is absurd.
What is absurd is making a definitive statement like that when you have no idea.
To know that SOI has no benefit, you'd need to compare two architecturaly-identical chips, alike in all respects save SOI.
Hammer runs at faster clockspeeds than Athlon, a simpler design but 130nm nevertheless. That's enough proof for me that SOI may provide some non-negligible benefit.
Who are you to say that Opteron would be just as fast without SOI?
Do you know for a fact that Prescott or Dothan wouldn't improve with SOI?
Or do you just take Intel's word for it?
fpg