satellites aren't cheap - who's going to pay for the navmap satellite(s)?
it's not like GM can just get NAVTEQ (or whoever) to upload their data to the XM or OnStar satellites to get it beamed down to your car
yeah, in the most basic sense that's kind of what would have to happen ... but in the real world that just isn't going to happen
NAVTEQ controls who gets access and how they get access to their map data
and on a side note... i'm pretty sure that the diagnostics sent from your car are not sent via satellite ... your car only has a download link from the satellite
the diagnostics are sent via text message (basically) ... think of amazon kindle's whisper net ... over the same cell connection that you talk to the onstar representatives