The problem appears to be affecting the palm rest and other areas of the computer on both the white 1.83GHz and 2.0GHz models. The first signs of the discoloration tend to appear after 2-3 weeks of use, according to posts in an ongoing thread on Apple's support forums.
It's unclear precisely what is causing issue -- some MacBook users speculate it may be a heat-related issue while others are chalking it up to a chemical reaction in the notebook's plastic enclosure.
Adding some support to the latter theory, one user reported that his Toshiba Satellite P30, which utilizes similar polycarbonate plastics, has also experienced the same form of discoloration.
Users have so far found little success in attempting to rectify the problem with a variety of off-the-shelf cleaning agents. According to posts on the Apple support forum, users have tried everything from iKlean and Windex to Ajax and Mr. Clean's Magic Eraser product.
"All the white MacBooks are showing discoloration on the trackpad button and palmrests, though I'm not sure if it's the same as people are describing here," wrote one poster who swung by the Apple Store Soho in order to investigate the issue further. "The ones at the Apple Store were just darkish looking, more gray than yellow."
As of Monday afternoon, Apple had yet to acknowledge the issue internally. However, AppleCare representatives have offered at least one MacBook owner the opportunity to send his notebook into the company's repair depot in Austin, Texas for closer examination.
Apple's computer warranties typically do not cover "cosmetic" issues, which is precisely how some of the company's customer support representatives have categorized the problem.
Apple began shipping the 13-inch widescreen notebooks less than one month ago.