i predict that whatever the ibooks and powerbooks get updated to, it'll be before the mad rush of laptop-for-school shopping (hopefully august for both of them to be updated by, meaning they probably won't be simultaneous). apple is really in a bind though for its laptops because of how close the ibooks and powerbooks are/can be in specs. We'll soon find out though is my feeling.
more specifically, i don't think apple will bump the ibook's cpu. i think what will get updated is the VRAM for sure but maybe the display (higher resolution), granted the powerbooks get the display update as well.