Not sure what to think of this... I got two iPhones this morning (two lines), and I was concerned enough about this issue to pick up two bumpers when checking out. However... neither phone seems to have this issue in the slightest. I've tried both with wifi on, with wifi off. covering it with my entire hand, just with a finger, nothing. Signal stayed constant.
Don't know how prevalent this is... or dare I say, if it's just mass hysteria, but it seems enough people are claiming it's an issue that it must be to some. However, it obviously doesn't affect them all.
Chalk up two more devices in the "working perfectly fine" category. (also as mentioned, picked mine up in store, not shipped. Maybe a run in one factory was bad, and that happened to go to some online preorders? Are in store purchasers seeing this same issue?)