I’m one of the lucky ones to get an iPhone 4 and also run into the issue where touching the stainless steel band/antenna on the sides causes signal degradation.
Here is what I can tell you:
The signal degradation happens when the overall signal is weak. I may have 5 bars, but as soon as I touch it, it drops. However, if I move around without touching the metal, say go outside, it is also 1 bar.
If the signal is strong, say near towers, freeways, or in an AT&T store, no manner of touching causes this issue. I held it with two fingers, four fingers, the palm, etc.
Update 2010-06-22: When connected to the microcell at 5 bars, no matter where/how I touch, the bars do not drop. However, if I am upstairs, about 50′ from the microcell, then touching the phone affects the bars reading.