Hey, I'm in the same boat you are. We just bought our 2007 LT1 Tahoe last week, and on our trade in we had an Alpine CD deck w/ XM. All of our paperwork plus the Chevy website states the XM is standard across the board on all models. We had even had a test drive an LS w/ XM. When we took delivery of the Tahoe, I specifically asked the dealer how to obtain the XM ID number for that vehicle so I could swap them out in my account. He didn't know but refered us to the XM paperwork w/ the vehicle. I couldn't find it so he checked and then brought us paperwork from onther vehicle. After taking delivery and arguing w/ XM for and hour & a half, it was detiermined that our Tahoe did not have XM. I believe the radio is XM capable, but it does not have the XM tuner and antenna. The sales person is working with the GM rep to correct this and says we're good to go, but have yet to have an exact timetable to fix it. I say go back to them and make them correct it, the tuner is a $200 part as far as I can tell from a gm parts direct website. False advertising is what it is, it should have been basic equipment. I was told depending on when in 2007 it was built it may or may not have XM.
Anyways, good luck and stay safe over there. Thanks for serving our country and come home soon to your family & Tahoe! You'll love driving it!