There are a couple of important differences to note with regards to comparing the G3/G4/G5 transition and moving from 68k machines to PPCs:
One, the architectural differences aren't as extreme between G4/G5 and 040/601; the PPCs actually had to emulate 68k code via the OS, and did so notably slower until the clockrates of the PPCs had advanced quite a bit. The G5, though it does require some optimization to function at its peak, uses the same instruction set as the G4 (and even G3, other than Altivec).
Since there is also no immediate benefit for most users of going 64 bit (only high end boxes that need >2GB RAM), Apple is, therefore, not under as much pressure to eliminate pre-G5 chips. They can keep the G4s around for quite a while longer at the low end without issue, if they want.
Second, as far as I know the 68k architecture had more or less topped out with the 040 series (unless you count the next generation 060s that never happened--remind you of anything with Moto?), and I don't remember hearing about any clock-rate advances with 040 series chips after the PPC shipped. The G4, or similar altivec-equipped IBM "G3" (basically a new flavor of G4), on the other hand, may well still have headroom.
IBM is still developing the line, since they're used in other situations, and presuming they can bring power consumption down further or scale the clock better, it *could* be a very long time before we see the end of the G4. After all, the G5 isn't wildly faster per clock in most operations; it's more the architectural headroom and other features that make it so attractive.
Put that all together, and I could easily imagine seeing "G4" iBooks (or maybe other, entirely new, machines) for quite a while yet. Maybe not, depending on how fast the cost and heat production of the G5 come down, but we'll see.
Anywhoo, the transitions are an interesting comparison, but there are some significant differences. Being (arguably) faster than PC users is certainly a familiar difference, though!
