It's unclear how widespread these problems are, but at the time of writing both Gizmodo and MacRumors have received more than two dozen reports from new iPhone 4 owners who say there is a discolored area on their screens, or an overall yellow hue to the entire display even after adjusting the phone's display settings. The images below aren't the best quality, but they do help illustrate the yellow spots and bands new owners are experiencing. In many cases, Apple has reportedly offered to replace units for customers with screen issues -- though they won't be getting their replacement today.
The other problem being widely reported involves the iPhone's wireless reception. Apparently there is a problem with the new antenna design -- which ironically enough was touted to boost the device's signal -- that sees reception drop considerably, sometimes from 5 to 1 bar or none at all, when users hold the metal ring around the iPhone 4. There's no definite answer as to what is causing the problem, but some say it happens when a user's hand connects the different metal sections of the antenna (it's broken into three pieces with small gaps in between).
A few hiccups are to be expected with any major product launch, particularly one as popular as the new iPhone. It remains to be seen how widespread these issues really are, and in the case of the iPhone's wireless reception, if it's strictly hardware related or if this is something can be addressed with a software patch.
Update: According to a forum post at AppleInsider, the yellowish screen issue is caused by a bonding agent used in the display's layers of glass. Apparently, Foxconn is shipping these products so quickly that the bonding agent has not yet fully dried following the manufacturing process. We can't confirm the legitimacy of the source or its claims, but supposedly the issue should resolve itself after a day or two of use.