I've had the wearing of keycap lettering on a series of Mac notebooks -- iBook G4, MacBook Pro, MacBook Air. On the G4, they repeatedly sent me, for free, a replacement keyboard (it simply pops out with tabs). The MBP was replaced as part of a general Apple Care servicing (Apple was incredible on that -- replacing all sorts of things, including new battery, logic board, keyboard, etc.). The Air, I'm just living with although my S, N, and M keys are dismal looking, especially when the backlight is on. (Oops! Just noticed that my F key is starting to wear away.)
I researched this like crazy over the years... some people blamed it on hand lotions, others on fingernails, and still others on the typing method (shades of "you're holding it wrong"). I'm a touch typist, do use hand lotion, and sometimes my nails aren't perfectly trimmed down. Never could get a handle on what was causing it; although in the iBook's case, it was an issue of the printed lettering on the top wearing away. On there others, especially the MBA, it seems to be a problem with the dark portion of the key flaking it off and ruining the form of the transparent letters.