I've been putting up with this defect ever since I switched over to the iOS 5.1. Having used iPhone since the original first-gen 16GB release (and 3 generations of iPhones after that) and not ever seeing this problem I'm quite sure of the timeframe of this change. I'd like to log a bug but it seems as if you can only do this if you're registered as an apple developer... is there another way? Although this issue isn't reproducible it occurs often enough that I see it at least a few times a week. The details of the problem are:
1. Network connectivity seems to drop permanently when trying to use the EDGE/3G/4G network in areas where the the signal strength is weak (this seems to be the trigger for the issue).
2. Resolving the issue requires either rebooting the iPhone or toggling Airplane Mode.
Other factors that my influence this:
1. I'm using AT&T's network so that may have a bearing on this particular issue.
2. I'm not switching to a wifi network while doing this- so it seems exclusively tied to the AT&T data network. However, I'd imagine that the bug may rear its head because the iPhone tries to switch to a wifi network when it doesn't see the data network available- note: I don't think the wifi dialog pops up when this happens so this is just speculation on my part.
3. I saw this start happening after upgrading to the 5.1 but not immediately after upgrading, but pretty soon afterwards. So, there could still be something else causing this- I provided the version 5.1 info because that may help Apple to try to figure out what changes introduced this problem so they can limit the scope of their investigation into this issue.
NOTE: I'm pretty sure this isn't hardware specific or specific to my phone. A friend has encountered the same issue. He told me that he works around the problem by toggling Airplane Mode (at the time I was rebooting my phone all the time which was pretty tedious). He saw the issue start happening around the same time I did.