Well, you’re not imagining those AT&T bars going down, it’s reality. As a matter of fact, here is Apple’s official statement on this particular issue:
Gripping any phone will result in some attenuation of its antenna performance, with certain places being worse than others depending on the placement of the antennas. This is a fact of life for every wireless phone. If you ever experience this on your iPhone 4, avoid gripping it in the lower left corner in a way that covers both sides of the black strip in the metal band, or simply use one of many available cases.
What does this boil down to? It means the damn Apple Bumper is actually a device you may want to get. I, for one have the Bumper on my iPhone 4 and am honestly not that impressed with the design of the thing. But per this reception issue, I’m thinking the Bumper is more function than fashion.