I sure hope something like that becomes more affordable in the years to come. Call me cheap, but $50/month for mobile data service is too expensive, unless you work all day from your car to justify it. Especially considering that broadband to the home can be significantly less expensive and is usually faster. I'm not sure the best way to make the distinction between the two types of usage, but I could imagine some amazing applications to tie the internet connection to a nav system like the MyGig.
Not only more real-time traffic updates as already advertised via Sirius, but also things like overlaying the radar image on top of the map. Or the ability to flip over to a Google Earth-like view for satellite images of your surroundings. Or more intelligent routing. In any case, here's hoping they do well and foster some price-reducing competition!