Personally, I don't think it will be too long before Mac do go to REV 3 and introduce a 3GHZ. A few months, if that.
Apart from the pressure from consumers (afterall, Steve Jobs is now over a year behind on his promise) graphics card vendors will be keen to phase in PCI Express and DDR2 as per the PC world. Also, the last update was June last year which means on Apple's production schedules the update(s) will have already been made and suppliers sourced / ready. On another thread someone posted an Apple PowerPC motherboard schematic from a suppliers website which would support this. I guess the limiting factor is IBM and the blip shortage of Nvidia 6800 cards they had over Christmas. However, I expect once the bulk of 2.5 orders are fulfilled we may start recieving news of an imminent update. A jump by 1GHZ to 3.5 GHZ would be nice, but 1. Their marketing deaprtment won't allow this and 2. I doubt they can get them stable enough at this clock speed yet without a liquid cooled wind farm running full tilt most the time.
Best
J