This looks like it's just a problem of firmware compatibility. If I was your friend I would just go back to Tiger until Leopard supports this model I'm afraid. You could try phoning Tascam of course, see if they can give any guidance. You'd think with Core Audio that all of this firmware hunting could be forgotten as a part of history. I had the same problems with a friends new iMac with the Edirol UA-25.
P.S - just checked for the drivers/firmware but was unable to come up with anything other than a release from 2006. Sorry. It looks like Leopard needs to be put back into it's cage for a while
not sure if anyone still needs info on this, but i sort of got the US-122L working in Leopard with the latest drivers from the tascam site. however, it is not showing up when i try to set up an aggregate device.
it shows up under the normal audio section in sys prefs and i can use it by itself in garageband, but i can't add it as an aggregate.
anyone have any ideas? i'd really like to be able to use this, plus the line-in for a total of 4 tracks....