Hi guys, just wish to clarify something..
The 2G/3G issue that the new iOS update is patching is as such:
- your iPad is showing 3G signal of at least 2 bars stably, but you'll still be shown an error message that's says it cannot "activate cellular data".
- your iPad is showing 3G signal of at least 2 bars stably, but you still cannot surf the internet on your ipad (you can try going to Google.com to see if there's any Internet connection on your iPad
Note that one consistent factor in all these issues is when your iPad is obviously showing decent and stable 3G signal but yet wouldn't allow you to connect to cellular data or surf the Internet. And this issue will only occur after your iPad had moved from an area with 3G coverage into an area with no 3G coverage, and then re-emerge into an area with 3G coverage.
If you noted your iPad's signal strength fluctuating for no reason, then the issue might lie more with your carrier. Try resetting the Network settings in your iOS settings.
Personally, I'm using the launch batch of iPad 3. Initially I'm facing the same issue, but after updating to the latest iOS, I'm no longer facing the 3G issue. Do note that I update to the latest iOS via iPad natively, not via iTunes.
Also, I'm using Singtel on my iPad, and StarHub on my iPhone. I can say that SingTel has a much much poorer 3G coverage than StarHub, which might explain why those on StarHub iOS 5.1 don't face the 3G issue as they don't transit between 3G/2G that often. For those who have updated to iOS 5.1.1 and are using Singtel, try resetting network settings. If not, try recovering your iPad via iTunes. If still got issues, maybe it's simply you don't have sufficient 3G SingTel coverage at your area, just like me.