This is some stuff I've found on the subject on other web sites. Don't know how much is fact and how much is conjecture.

"Its official. My XM rep told me at work you'll see this happen by the end of 07.
No big changes though unless you want changes.
For instance, if you have XM and you want Sirius channels you can add them a la carte.
Choose what channels you want and you pay a little more for that service.
Kind of like cable TV."

SO:

1) Rates will remain the same
2) You will keep your main provider. If you have XM, you still have XM with no changes.
UNLESS: You want to add channels to your XM receiver that only Sirius now offers. (and vice versa I assume.)
3)You will be charged accordingly in a bracket with how many channels you want.
4)ALL Sirius and XM receivers made will be able to handle this change. No need to upgrade to new receiver is necessary.
5)If you prepaid service your prepayment will go unchanged.

My understand is that even though they advertise "commercial free." XM is not commercial free.
All or most of the "talk" channels, sports, news, etc have commercial interruptions. MOST of the MUSIC channels are commercial free.