I just posted over on Projects about a Kenwood Changer interface to do exactly this with a Kenwood Sirius tuner. It looks like the hardware interface will be much easier for XM, but with a little forethought, the software side of this could support both services.

The XM direct looks like a straight serial interface, while the best option for Sirius is probably the Kenwood tuner, which lives on a Kenwood changer bus.

I prefer Sirius' programming, but if we can't make Sirius work, I'd look hard at XM. (Not interested in a Sirius vs. XM flame war, either.)

So, count me as interested.

EDIT:
I've now found that the Sirius Starbase has a normal serial interface between the display and the trunk box, so this should work as well as the XMDirect. I think we need level shifters for both of these, but that's easy.

So, do we have any interest from a developer? I'm much more of a hardware/IT guy than an SW developer, and haven't looked at what it takes to run the display (a la gpsapp), but I'm certainly willing to work on the back-end such that we could control either flavor of tuner.