I don't think it's an issue with a specific network, because it seems to be affecting people with different carriers in different areas of the world - some are in England, some in the U.S., I'm in Canada.
It has to be something to do with the new iOS5. What exactly, I don't know, but it seems like the phone gets overloarded with information after a while and something prevents the phone from connecting to the 3G network (even when it says it's connected, in some cases).
The only thing that worked for me was switching off the 3G network in my settings. This allowed my phone (a 3Gs) to function as a phone and get texts - on the slower o or E network - however connecting to the internet or recieveing any other data is obviously torturously slow on those networks. But at least I could get phonecalls on my phone. After turning on 3G again, my phone would work normally for a bit, but then after a period of time - sometimes minutes, sometimes hours - it would revert back to not being able to connect to the 3G network for anything. No calls, sms or internet. Nada.
I tried everything, many times. Reboots, switching off and on, airplane mode, changing a whack of setting, restoring the phone as a new phone, doing a DFU recovery/restore, nothing worked. So Apple agreed to replace my phone. It works fine - on the old 4.3.5 OS.
I'm staying away from the new iOS5 for a while. It seems like this problem is surfacing for more and more people, but not enough for Apple to do something about it - yet. They will after a while. I'm sure there is a minimum number of complaints to Apple that trigger a software fix. So I urge everyone having this problem to call Apple and bug them. The more people who do, the faster they'll fix it.