There is similar, much longer, thread on this. We have generally come to the conclusion that it is a bug in iOS 5.1.
I have spoken to Apple support who got me to reset the ipad as new rather than restore and it made no difference. Now they want me to get a new SIM card from t-Mobile saying that there are new cards... Don't believe it myself as quite a lot of people are suffering this and on different networks.
What if have found is that it seems to be when swtiching from wifi to 3G, e.g after using wifi at home then going out. So far I have found that leaving mobile data off until you want to use it and then switchig off again seems to make it work without having to restart or reset network settings...