IMO, it's gonna be awhile. The iBook was actually updated pretty recently. Only more recent updates were xServes and PBooks. (iPods don't count since they are a separate division from Mac hardware nowadays)
Before that, expect updates or EOL for the eMac, a speed bump and maybe bluetooth 2.0 for the iMac, and *something* happening with the PowerMacs. I'd expect a new 17" cinema display first too.
None of these things are required for Apple to update the iBook line, but in the past year or two, Apple's really started spreading out its updates. I imagine it's related to the run-up in the stock...you've got to beat expectations every quarter. That means surprising people all the time. It's like bloggers who line up posts in advance but only publish one a day. I'm waiting for a PowerMac, me'self, and because of that I'm been obsessively reading ever rumor site/forum. From that, I get that the refreshes I listed above are queued up.
I haven't read a thing about an iBook update. With the GPU they got in October, they're already set for Tiger, so I don't think that would be an impetus either. My guess? Either they'll stay where they are until a G5 can be crammed into a PowerBook (not any time soon), or Apple will come out with a modest bump (a la last month's alBooks) in time for the mid-summer .edu rush.