i think the iBook is more likely than the commentary here would lead to believe, because the last update was much smaller than anticipated and hinted at. the last update appeared to me to be a small "hold over" update until some larger one could be prepped and made - when is macworld tokyo? i thought that was the traditional portable announcement spot.
at any rate, i think the iBooks are in need of an upgrade and can only hope this summer will see it. the ti books are fine capabilities wise they are just too damn expensive - they will not drop the price i bet, so i guess they have to upgrade that one!
i know some people that work at the Apple store here in cupertino, and some say the iBook is better than the Titanium Powerbook in many ways. it is much tougher for one thing - the drop test kills TiBooks dead, while an iBook has a 50/50 chance of surviving. the TiBooks is apparently so thin and has so little space inside it that the littlest damage is major. the plastic case on the iBooks is supposed to help a lot with it's flexibility.
as for the screen size discussion, personally i am going for the smaller iBook. the larger screens make the "portable" part invalid i think... the idea is a small supplement to a major home machine, and the main thing is portability to me, so those little iBooks look the best.
of course , the main complaint is the hard drives - they are so slow! at least the Ti you can get a large one that is 5400rpm, but the iBooks are limited to 4500rpm! and replacing them is a nightmare. so i guess for pro music or something an external firewire drive is the best solution...