ZamAudio provides an unofficial Digidesign Mbox 2 driver for linux (kernel module):

• Digidesign MBox2 driver

Unfortunately the patch is not part of ALSA … I expect license problems or something like that (http://www.zamaudio.com/?p=233).
The mbox2 use their own proprietary protocol and not a generic usb-audio one.
We all should switch to some other device – motu, maudio etc – and thank avid/digidesign for their support (!).