I'm in the same predicament. Here are a few Ideas I had. Anyone is willing to comment upon them.
I've been using ZTerm on the Mac, Term (available from Aminet) on the Amiga, and just transfering via z-modem. You should be able to get a USB to serial adapter for you iBook.
Another option which I would like to explore is hooking up an HFS formatted hard drive to the Mac, dumping files onto it, then connecting the hard drive to the Amiga (assuming that I can get the accelerator card working for Amiga OS 3.5).
Third option is to write stuff to a CD and read it from an external CD on my Amiga.