It's a yellowing of the plastic it is not cleanable.
Who wants to have to return their laptop for silly stuff like this?
Not me.
Yellowing of the plastic? What are you doing, storing it in a ziplock bag filled with cigarette smoke? Rubbing mustard all over it? Using it while engaging in golden showers?
I've owned 2 iBooks over the past 5 years, and now I have a MacBook, and yellowing is something that has never happened to me. I've never even seen it on any of my friend's or family's MacBooks or iBooks. Or on any of the MacBooks in any store that are being used as a floor model.
If you take care of it, it should always look great. My iBooks still look relatively new, except for a little discolouration on the trackpads. They are as white as they were when I bought them.
By the way, I don't do anything special to keep my MacBook clean- I just make sure my hands don't have fingerpaint on them, I don't store my MacBook by the deli meats, or in the bathroom. I just wipe it clean every couple of days with the microfibre cloth that was in the box.