That's the sobriquet du jour for a screen problem that some iPhone 4S users say affects their new smartphone. Several messages left on Apple's support forum reporting that their units' screens had a yellowish tint.
CNET noted that similar complaints attended the rollout of previous Apple products, including the iPhone 3G and the iPad. Also, some iPhone 4 users complained about encountering a yellow discolored area or yellow tint on the screen. In most cases, Apple offered to replace the phone.
CNET has more about the issue here.