My car ('06 Buick LeSabre) has buttons on the steering wheel that control the volume and band on the factory radio that came installed in the vehicle. Mrs. H and I would like to replace that with an XM-ready aftermarket radio. Would the buttons on the steering wheel still work with the aftermarket radio?
Can be done, though I would assume this is not guaranteed in all cases:
It can be done; but you’d need to look at the specific radio you want to put in. I’d suggest looking at Crutchfield; they’re real good with that type of thing, they even have custom install kits to fit most vehicles,
My car – 2011 Dodge Caliber – required an adapter module since apparently some of its controls were contained in the radio. The module used depended on whether the car had steering wheel mounted controls (mine didn’t). YMM, of course, V.
The two car radios I’ve purchased have cables marked for steering wheel control, and have adapters available by make and year of car.
I have an older Volvo where the kid who owned it before had a aftermarket stereo put in, and no, the steering wheel buttons don’t work. 
I recently installed a new Boss stereo in my wife’s 2004 Prius, and like the others say, I had to get a separate adapter module to connect the steering wheel buttons to the new stereo. Not a big deal, just make sure you get the right adapter for your car and stereo.
This was the same for my 2012 Dodge. There were functions such as “low fuel indicator” controlled by the radio CPU and replacing the radio made the car lose that function. And to use the buttons on the steering wheel, an additional module that sat behind the radio head was needed.