To clarify here a bit, Apple is moving the Mac to Intel processors over the next year or two. They've formally announced this and all that. They haven't announced what will be moving to Intel and at what time, though at least for a while there were rumors that the first Intel-powered Macs would be launched in January (those rumors have since died down). Since Apple's going to be using Intel's Yonah (dual-core Pentium M, launches in January), Conroe ("Pentium 5", the Pentium M-based successor to the Pentium 4, supposedly launches in the second half of 2006), and Merom (the notebook version of Conroe, 2H '06) CPUs in the Intel-based Macs, not the current Pentium 4-derived desktop chips, January's the earliest Intel-based Macs could launch.
Since the Mac Mini and the iBook are the Macs with the most serious problems with underpowered CPUs, and because the low-power Yonah chip is a better fit for them than the more powerful iMacs, PowerBooks, and PowerMacs, speculation is that they'll get the Intel treatment first.