The buttons aren't prone to corrosion or discoloration. However depending on the nature or your use the aluminium looking paint they have can wear away.
I've used mine naked for a long time and had no problems. My friend however has had his wear away. I can only assume that it's happened because he's pulled the phone out of his pocket and pressed the buttons 50 times a day. Or he likes to rub them in his pocket.
This chrome backed watched will give you an idea what could happen.
Here the lower part of the backplate has been rubbed away to reveal a pinky copper layer. You could expect something similar. I'll post a photo up next time I meet him.