A lot of the powder coating had rubbed off the palmrest areas and the strip along the bottom of the keyboard, presumably just due to contact/sweat. Some of my frequently-used keys were looking a bit shabby too.
It didn't look pretty, but it didn't stop me buying another one because I'm more interested in what my Mac can do that what it looks like. This time, however, I have applied a clear "Wristguardz" protector from
http://applepeelz.com, and so far it's working great. (It's also quite possible that Apple have improved their processes since then, to prevent it happening in the first place.)