Sirius hopes many dish network subscribers will really like their channels, and then go out and buy the hardware/subscription for their car. This will certainly happen, to some extent. However, it could also mean increased subs for XM--as many dish customers who now want satellite radio for their car, will not blindly go out and automatically buy Sirius, but they will do some research, and just might want XM. So, this is really a great thing for satellite radio, in general.
I have XM in my car, and have had sirius in my home for several months now. I absolutely love both services. I think its tremendous that Dish has Sirius, as the music is superior to the dreaded "musak". I'd trade all the Musak channels for one HD channel.