In my “spare” time (hah!), I hack on Arduino a bit. Mostly because there are tons and tons of 3rd party libraries that make hacking up a hardware solution mostly a bit of soldering followed by gluing together some pre-made software bits. The Arduino IDE is Java based and… well… not terribly awesome (to be fair — it isn’t awful, just quite lacking beyond the basics).
With the release of Mountain Lion, most Arduino installations were broken. Fortunately, this can be fixed by grabbing the latest bits from here and there.
If all went well, you should see the device show up in /dev/ as something like