As I understand it, the xm radio is prewired as part of the Nav system - if you get the nav system, you automatically get the xm radio (without the subscription). The screen would probably do double duty between mapping and xm station information.
If that is the case, a person without a navi system -by ordering an xm radio system - are asking Honda to install a seperate xm satellite system into the truck, hence the cost. Also, I heard from someone (not sure if it is true) that the xm antenna and the navi antenna share the same "hump" on the roof. If true, that is something else that Honda would have to include when installing an xm in a non-navi truck.
In any event you could always go aftermarket on xm.
Personally I'm going to get the navi system package.
I believe that the XM Radio does share 'the hump' on the roof with the regualr radio and the GPS antenna. At least I think it is that way in my Odyssey (I have the Nav and XM radio). I posted some pictures in another thread of the various nav system screens. I can say from personal experience that the Nav system in the Odyssey displays nav info, XM radio info, regular radio info, CD/DVD info, and system settings info.
Edit: Here is a link to the thread where I posted pictures of the Nav screen from my Odyssey.