Actually since Intel is moving away from the "Netburst" Pentium 4 achitecture, they'll be moving to a new one called "Conroe" which means short pipelines in the processor and they will reduce their speed as well to reduce power consumption (mainly because the Pentium 4's consumed some 250+ WATTS when under load!!) The shorter pipeline will allow them to work harder, but at the same time reduce clock speed.
Right now, Pentium has a 31-stage pipeline, G5's have a 23 (I'm pretty sure) and Athlon has a 13 still. That's why a 2.6 Athlon FX-57 will speed by a Pentium 4 EE 3.73 GHZ processor (Pentium 670) However, Intel will go back and reduce speeds, so we probably won't be getting 3.73 GHZ.
It just depends, we will break the 3GHZ mark, it'll just be a little harder. Good thing is, we'll have huge L2 caches which will be good for audio/video encoding. I'm sure Leopard is going to make good use of the extra power. I'm surely excited for Mac, I can't wait to see what goes on, if anyone's interested in keeping up with updated news for Intel's new architecture, there's always information at Tom's Hardware or Anandtech (mostly PC news)