ADB was a great connection and died a sad death. The last mac to have an ADB port was the B&W G3 tower.
I have read on xlr8yourmac.com that OSX only likes ADB if its built in to the mac and adapters are sketchy at best.
Actually, there were later portables whose trackpads used ADB, just not the standard connectors. Which makes it odd that OS X doesn't like other ADB devices. Must be the USB adaptor bit that causes the problem.
Gives me an idea though: Re house a laptop which has an ADB trackpad, only swap out the trackpad for a tablet..........
Apparently even the early Alu PowerBook G4s had ADB pads. Mactracker doesn't list them for iBooks, but I think the clamshells definietely had ADV, probably some of the earlier white G3s too.