Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Apple's iOS 5.1 Beta 3 Brings Back The 3G Toggle –
Apple has just released iOS 5.1 beta 3 to developers. You can get the download OTA (Over the air) if you are already running iOS 5.1 beta 2 on your device , beta 3 is yet available to download from the developers portal. One of the biggest things we have found so far is it does bring back the toggle setting to allow users to turn on or off 3G , This was not included in iOS 5.