I gave 3.1 a go with a jailbroken ipsw and unlocked it. I got a blank carrier. which was a little annoying but sorted it via a plist edit, also having an issue where if my handset is turned off the apn details for my carrier get wiped everytime I may be going back to 3.0.1 within less than 24 hrs of 3.1 being put on
The blank carrier is a simple fix. You need iFile app to do it right from the iphone. https://forums.macrumors.com/threads/798473/