I just had mine replaced under warranty.
They were so into it they ordered a whole new unit instead of the buttons. I guess the dealer did not care since it's under warranty.
Had my replaced about 2 months ago. Dealer replaced the whole unit, no problem, all under warranty. I agree with CIWS, the dealer knows it's a common problem.
I've had a couple. My first radio just had chipping buttons. They replaced it and the second one died the second day so they put the old one back in. This new one...it randomly restarts/reboots and says "driver 1" in the middle of driving. I imagine they will be replacing this one too.
It can depend on the dealer.
The knobs are replaceable by ordering them through GM.
We replace the buttons for our customers, if there is nothing wrong with the radio, no reason to replace the whole unit, it helps keep our warranty costs down and lets us do more for you in the long run.
Most dealers don't know how to get the buttons so they replace the whole unit... but as mentioned this can cause problems every now and then.
The faceplates can be serviced also.
Some dealers will call it normal wear or abuse, so it is up to whoever you take it to, really.