That completely depends on how you care for it and the environment where you use it. If you put it on dirty surfaces or use it with dirty hands, it gets dirty, like anything else.
not really a rev A product and the rubber on the bottom will probably get a bit dirty with time, and the white surface will simply scratch, no yellowing there, I guess.
go to the store and look at one, either apple store or bestbuy. Any display they have is probobly from when they first came out and so you will see what it looks like if it gets dirty since tons of people come into the store and play with them