One iPhone 4 demonstrated the issue everytime it was held in our left hand (as a right-handed person is apt to do) so that our palm was essentially bridging the two antennas. You can see that in the video after the break. Bridging the two with a finger tip, however, didn't cause any issues with the reported reception. If we had to guess, we'd say that our conductive skin was acting to detune the antenna -- in fact, we've already managed to slowly kill two calls that way so it's not just an issue with the software erroneously reporting an incorrect signal strength. That said, we had no issues when Apple's $29 rubber bumper accessory (given to us free for standing in line) was attached, creating a buffer between our palm and the antennas. Our second UK-purchased iPhone 4 was fine, showing none of these handling symptoms. See the video evidence after the break including Insanely Great Mac's version which got us to worrying in the first place.
P.S. Don't forget to take our poll and let us know if you're seeing both the yellow spots / stripes and reception issues. Unfortunately, we're suffering from both flaws which is not a good sign for quality control on this first batch of Apple handsets.
P.P.S. Since some of you are asking, our review unit showed none of these issues.
Update: Apple responds to the issue, and boy, it's a doozy.