I don't think it's a dumb name. It's a strong term that lets you know they are "serious" about their service.. I relate XM to the likes of FM/AM radio. Why would you want to be related to traditional services when you are nothing like them?
Here are some facts on the word Sirius.
Sirius is the brightest star in the constellation Canis Major, and is also the brightest star we can see in the northern hemisphere. In addition to these two facts, there are also some interesting tidbits about this star which are less well-known.
The ancient Egyptians called Sirius the 'dog star', after their god Osirus, whose head in pictograms resembled that of a dog. In Egypt, Sirius shines for most of the summer, and since it is such a bright star, the Egyptians actually believed that the additional light from this nearby star was responsible for the summer heat. This of course is not true. However the origin of the phrase 'the dog days of summer' comes from this ancient belief -- the 'dog star' being the root of this common saying!