Could be an interesting legal question if and when the merger goes down. Nothing has been discussed regarding a new name of the combined entities. It could all be XM, Sirius, or even some new name.
If what you now have as a Sirius lifetime membership suddenly is no longer "Sirius Satellite", then its "lifetime" could be all over. I don't know that any figures are public knowledge of how many lifetimers there are, but it could be interesting if hundreds of thousands of people are terminated from memberships to a "no longer existant" company or service.