Has anyone tried restoring through iTunes? What if this is all because the earlier phones had a revision of 4.0 before some tweak was made?
How about restoring from iTunes and NOT restoring your backup? What if it's some fluke with some code that is brought over from the 3GS?
It's an extremely unlikely fix action but really, it's worth trying for sure.
When I had the 4.0GM on my 3GS and had restored from backup, I had weird signal issues, when I updated to the released 4.0 (with the same exact build number) and did not restore from backup, my signal is rock solid from the testing I just did.
If I had gotten my iPhone 4 today I'd test this out myself...but I'm number 22 in line. lol
*Sigh
It is proven 100% to be caused by bridging the two antennas.
All we need to do us wait for Apple to change the antennas outer material to something that cannot be bridged.
Those who have the iPhone 4, get a Bumper or plastic case or which ever you want, just cover the antennas.