I have a DataHand keyboard connected to a KeyStone PS/2->ADB adapter on the ADB port of a B&W G3, and it exhibits odd behavior in XFree86 (on Mac OS X 10.1.4). In particular, characters appear multiple times when typed quickly, as though there is a buffering problem with ADB - typing "hello" can result in something like "heleloo" appearing in the window. This only occurs with X-Windows (it happens with Xtools too), but not with any other Mac OS X application, so my first suspect has to be something in the X11 libraries.
The truly odd thing about it is that it does _not_ occur, on the same machine, if I plug the KeyStone PS/2->ADB adapter into a Griffin Technologies iMate ADB ->USB adapter and use the keyboard. If I do that, the keyboard behaves normally - which would solve the problem nicely, except that I only have one iMate and I usually use that on my laptop at work (which doesn't have a native ADB port).