"Hi Mike, It's been a while, but I thought I'd pass this along to you as it's worked for me as a stop-gap fix for the antenna issues with iPhone4. First off, I'm definitely experiencing the sharp drop-off in signal when gripping the phone normally (in my left hand, I'm right-handed).

I'm going to get some bumpers from Apple to fix this with 'style,' but in the meantime this has worked for me - use a standard wide rubber band and wrap it around the bezel/antenna of the phone.
So long as I'm not gripping the phone too tightly, this has significantly helped the problem. Pic attached.
Otherwise thrilled with the phone.

(I asked him about the "bumpers" he mentioned.)
If you haven't seen em yet, here are the bumpers:
Apple iPhone 4 Bumper - Black ($29 US)
I'm pretty sure that they are Apple's attempt to mitigate fallout from the antenna issue.
For what it's worth, I drove around this morning and found that when I was in areas with very strong coverage I could not get the signal to drop. So, it appears to be localized to areas where AT&T has sketchy service (unfortunately, like at my home), coupled with antenna issues and gripping the phone 'normally'.
Take care, Matt"

(Matt's no-bars to full bars rubber band mod)