Here's a thought - would you be able to afford a G5 PB if/when they come out? Figure that a 15" won't cost any less than the current - $1999, and in fact probably will initially carry at least a 33% premium like the G5s do (now figure about $2600).
I'd say that it'd be silly to pick up a G4 PM system right now if one was craving the power of the G5 and could afford it. However, we know they're coming.
G5 PBs, on the other hand, are a mystery. I doubt they'll make it into portables in the very near future - did you see the massive cooling setup for those things? *Several* dedicated fans? Massive heatsinks? Perhaps the 1.4ghz model is relatively cool, but we just don't know, do we?
You have to weigh against - how much happier would you be with a new machine? Do you do work on it? How much time do you waste with the old machine (time == money)? How much can you get for the iBook if you sell it used, now? If new G5 PBs come out in a year, would you be willing to sell your NEW iBook/PB, and take a bit of a hit (Macs hold their value EXTREMELY well, especially the portables)?
If I had the money and the need, I'd pick up a new system today. No point suffering through with a painfully sluggish machine for what could be a year or more (as someone else mentioned, it took 1.5 years for G4 PBs to show up)
Rip