We came across a couple of forums featuring the same concern. It seems white MacBooks have a tendency to lose its immaculate color and turn a little yellowish around the trackpad and keyboard area. (These areas have matte finish to begin with which does look attractive but  a lot harder to clean than a smooth surface.)
 
The discoloration starts to appear anywhere from two to three weeks of use. It doesn't affect its performance but it is kind of an eye sore. It makes the owners look like they lack personal hygiene or something. The owner of the MacBook in the image says:

"The spots don't wash off, I've only had the MacBook for two weeks...can't be my hands. I don't smoke, don't use latex, etc. And for those who worry about my hygiene habits, thanks. I wash my hands." You're welcome.

The stains, in fact, are not even caused by dirt because they appear resistant to Windex, the universal formula that will wipe clean anything dirty except a guilty conscience. The discoloration is also immune to iKlear, non-bleach Chlorox wipes, 3M Magic Eraser and plain wet cotton towel.

Other concerned Apple People have forwarded some possible "cure" but as one discolored, err, disgruntled, owner observed, "Its seems like more than just a surface "stain" and seems like a change in the plastic itself, which is worrisome. I wouldn't mind so much, but i'm just concerned how this will look months down the road."

Apple has yet to address this issue.