If the Power Macs are going to reach 3 GHz, you can sure as hell bet that it won't be announced next week, when Steve could be announcing it himself on stage at the WWDC keynote.
This information leads me to believe that we will see something like 2.6 GHz Power Macs, not lackluster, but certainly not 3 GHz. Apple will not want to announce what could be seen as a "failure" at WWDC, so they will announce them before or after.
If they announce after WWDC, like in July, the entire Mac world would erupt in anger and confusion right after the keynote; so they have to do it before.
My only prediction: If this website is correct, and new Power Macs are released on this Monday or Tuesday, they will not be at 3 GHz.