Background info: Last June I purchased an iphone 4 from ATT and in September I canceled my contract because I moved to the UK. In the meantime I jailbroke/unlocked my iPhone 4.1 so I could use it on the O2 network. This past December I was having wifi issues and restored my iphone which got locked back to ATT. I took it to a UK apple store and they replaced my iPhone with a UK unlocked version. They inserted my O2 sim card, and everything worked flawlessly. I even symced my iPhone in itunes, and it worked fine on O2. BTW: This was an iPhone model MC605B Fast forward to last week. Since ios 4.3 came out I decided to upgrade the software on the iphone. Once it finished, the phone would not activate, and itunes kept telling me that I had an invalid sim card installed. The only was I able to get past the itunes screen on the phone display was by putting in my old ATT sim card. After the phone activated I put my O2 sim card back in, but it would not get a signal, while itunes kept telling me I had an invalid SIM. I restored the phone back to 4.2.1, however it remained locked to ATT. After I called Apple Support US, they mentioned that my phone was provisioned for ATT (and supported on other EU carriers). They told me to try a different SIM, but it still didn't work. Off I went to a local apple store, and they replaced my iphone for "No Service". I currently have an iPhone 4 on ios 4.2.1 MC605B working fine on the O2 network. I called apple again to see if a restore to 4.3 would result in an ATT locked iPhone; however, they said my phone was provisioned on the O2 network. I have a feeling that if I restore to 4.3, I'm going to end up in the same situation I did before. Any ideas?