Gap between rear glass an antenna
I myself have this issue. It is a defect, & if you have it, Apple does replace these units.
I've done quite a bit of web searching to find info on this topic. There's not a whole lot of info out there. But from the few mentions that I've seen, and from my discussions with Apple techs it is a real issue that effects some iPhone 4 made early in assembly. The error was later found and corrected so iPhones made after are not affected. Supposedly only a small number of units have this issue, but it can cause problems in a few ways.
1. It creates a pivot point on the rear glass that could given the 'right' conditions, like a drop, cause the glass to break. Where if it were properly secured a drop won't break the strengthened glass.
2. It allows a space where particulate, dust, even skin cells into the internal components of the iPhone. There have been claims already that this gap is connected to 'scratchy' top mics. As well as foreign 'objects' in the rear camera lens.
3. It could allow water, or even moisture into the internal components. Even triggering the water sensors in the mic port.
For me it hasn't been an issue just yet. The phone functions perfectly. It's by far the best I have ever seen. But if this defect slipped thru quality control, what else can I look forward to surprising me in the future? For now I have her in the free Griffin case Apple bought for me, and I take good care not to drop it. I will be exchanging it at the Apple Store when I can get up there. (100+ miles away) But I think that is a small price to pay for such cutting edge design & technology in such a small package.
FYI How to decode the iPhone’s serial number and find out when your iPhone 4 was assembled:
UU = Factory and Machine ID
V = Year
WW = Production Week
XXX = Unique Identifier
YY = Color (A4=black)
Z = size (S=16GB, T=32GB)
Mine was manufactured in week 29 and does have this issue, I've also heard it exists in week 27 as well.