APLX is an APL2 implementation for various systems: Linux, Windows, Mac OS X. Unfortunately, it's a bit expensive - there's only a cheaper Personal version for Linux, otherwise there's only an Evaluation version for Mac. And how they solved the problem with the APL special characters is not mentioned there - presumably via combined keys or something similar. I wouldn't know if there is an APL-USB keyboard.
Somehow, I would like to play around with APL again - the language is wonderfully crazy and has a few corners that even today (except in languages like J and K of course, conditionally also in Sisal) can find their match. Specifically, the ability to process entire arrays of values at once and combine them with powerful Higher-Order functions is really exciting.
J is now also available in an OS X version, unfortunately only from 10.3 onwards. Would be another alternative, J can be considered as APL-in-ASCII (although it offers a whole range of improvements in language theory compared to APL).