It's never pointless to imagine about the future.
Sure... no one knows, but I would imagine, the upgrades will be about 4 times better than what has taken place over the past 2 years of iBook evolution. Even the duration between the iBook 466 SE to the last G3 model... it took just over 2 years to grow 434mhz, go through several case revisions, chips, boards, etc...
Now we have a new iBook class with the G4... which I imagine could still be the standard for the iBook in just under 2 years, but at least a 2.2Ghz chip, insanely large harddrive (which may not be neccessary as compression gets better) 2GIG Ram... superdrives will be the standard across the apple line... basically... a current powerbook on steroids.