Supposedly Apple is trying to maintain a 5-month lifespan on all products, which means that after an update, there's probably going to be a 5-month period before it gets a serious rev. However, I would venture to guess that the iBook will probably not get updated until July (6 months).
Depends what you want to do. If you absolutely must have OS X running decently or faster than what you currently have (I dunno if it's decent on your Powerbook or not), then you could feasibly spring for a new iBook. If you can live with your PowerBook for a few months yet, then you could wait.
This is my philosophy: you could wait forever in getting a new computer. Even after an update, there could be another update in as little as two months (the Powerbook and iBook both got updated in October, and then the PowerBook got updated AGAIN in December and the iBook in January). So unless a big trade show is coming up, you should spring for what's available.
In this case, MacWorld Tokyo is coming up in about a week, so I suggest you wait until after then. But after that, you can safely spring for an iBook.
You're inevitably going to be disappointed when a new revision comes out, but it's always a guessing game as to when a new model is coming out, so your judgment is as good as mine.