I am pretty sure the ibook uses the "superslim" model notebook harddisks. That excludes the 48gig notebook drives, as they are not "superslim" - i think the largest that would qualify is the 40gb toshiba disk.
That said, i recently saw an announcement of a 60gb notebook drive (not superslim), but that could signal a coming shift that would push things like the 48gig drive to a smaller form factor.