I think all Mac Laptops manufactured before late 2004/early 2005 use internal ADB busses for the trackpad, that includes the early G4 iBooks.

I think only the USB trackpads support 2 finger scrolling, so if the laptop doesn't have this feature it's probably using ADB.

I connected to the internal USB bus when I modded a Wallstreet PowerBook (http://flux.openobjectproject.org/?p=8), it was fairly straightforward. (Waragainstsleep - I like your tablet idea)