iOS 5.1 and no 3G data?

Just a tip - if you upgrade to iOS 5.1 and suddenly your 3G data stops
working, go into settings and check the APN. It will have gone back to
its default, and if that was incorrect in the first place (such as on
my TPG SIM) then it will stop working.

In my case I changed it from CONNECTME to INTERNET and it all started
working again.

Andrew Calvin 2012