The same thing happened to me.
When I plug the US-122 in, the entry "Vendor-Specific Device" appears in the USB Device Tree (in the Hardward:USB section of System Information), but the US-122's USB light does not go on. When I plug in a US-122L, "US-122L" appears in the USB Device Tree, and the USB light on the US-122L goes on.
On a Mac running 10.6, the same things appear in the USB Device Tree, but the USB lights come on for both devices. However, the USBlight on the US-122L comes on immediately when the device is plugged in, and the USB light on the US-122 only comes on after the driver has been loaded.
I've filed a support ticket with Tascam. I'll report what they tell me. (I don't expect them to be much help; in the past, when I've had compatibility problems with the US-122, they've just told me "it's not supported" --- even in cases where I subsequently got it to work.)