Yes, probably on release. Still, we know which models will be supported and it can't hurt to start shopping around. I look forward to having a mobile system.
Well, if you remember, I think that the MorphOS Dev. Team listed fewer G4 PowerMac models than they actually ended up supporting upon initial release of support for the G4 PowerMac models, so the same may be true for the G4 PowerBooks, when support is finally released. Also, there is one last model of the G4 iBook that could be supported, as it has USB keyboard and trackpad, just like the G4 PowerBook models that are going to be supported. Due to the differences in the G4 PowerBook models, there won't be as many different models supported, like the G4 PowerMacs, but there might be one or two more models than have already been announced by a few of the developers working on it. Has there even been an "Official" announcement on the MorphOS website about G4 PowerBook support? We know it is coming, but last time I looked at the official MorphOS Team site, I did not see anything about PowerBooks.
Given the speed of the last G4 PowerBooks (1.67GHz) and the better graphics cards than previously supported MorphOS2.x systems, they are going to be great, screaming fast MorphOS2.x systems and portability is just icing on the cake.