I had a similar problem with O2. Throughout my two year contract with them and an iPhone 3GS I endured a very weak signal along the lines you describe. At the end of my contract I told them I was leaving and was assured that the iPhone 4 would not have the same problem and I was persuaded to take out a new contract with the proviso that I could cancel if it didn't work out. When I received the new phone, it was exactly the same and I returned it immediately and cancelled (you have an option to do this for, I think 14 days). I took up a contract with Vodafone and after some three months I have only once failed to get a 3G signal and this recovered when I switched the phone off and then on again. I think it fair to say that the providers seem to have black spots where they can't achieve a decent signal even though the post code denotes the opposite. In such circumstances Vodafone have a system you can revert to whereby they can obtain a signal through your wireless router ( providing you have one of course) but I haven't found the need to use it.