Yes, the Lexicon Omega is declared to be supported by the last Debian's releases; in fact, essentially by Alsa (http://www.alsa-project.org/main/index.php/Matrix:Vendor-Lexicon
Unfortunately it seems that there are difficulties with the actual-squeeze version of alsa (1.0.23).
The device is recognized by any audio application for playback; but there's no mixer and Ardour cannot record single channels through each of the four (since it's a USB 1) buses. [see the attachment]
It has been suggested to compile and install a more recent version of Alsa (sorry, this is an Italian forum http://forum.debianizzati.org/viewtopic.php?f=17&t=43489#p132125
but I think the commands are quite self-explanatory).
I'm going to try such a solution... hoping the best.
"See" you soon,