I expect at least 1 or 2 more speed bumps to the G3 in the iBook before the switch is made. By that time (12-18 months?), the TiBook will have some form of G5 in it, at which point the question we'll start seeing is "Hi, I was wondering if you think there will ever be a G5 ibook..."
I think not. The G3 will still be boosted to about 1.2 or more GHz by IBM. That'll be used in consumer machines by Apple. When the G5 will take over in professional machines, there'll also be a low power consuming version. The G4 is too hot for powerbooks anyway, so I guess the iBook will jump from the G3 to the G5 directly at its time.
Before I found out about the G5-lite or whatever... I was wondering how Apple was going to continue with it's high-end/low-end processor stepping for its little tiered product system... since the G5 was going to be 64 bit and obviously perpetuating the era of fat binaries is a bad, bad thing...
But low-end G5's solve that problem quite nicely. The only problem with that is explaining to people the difference in the chips.