I did this too Anton, it worked for 3 days and I really thought it was fixed, but is wasn't.
The problem is definately caused when the iPad switches between 2G (or Edge) and 3G. That locks up cellular data on the Globalcomm multi chip. If you live in a country where 3G coverage is not so great and 2G is still common, you’ll notice the problem a lot more than when in other countries. If you live in a major city where you mostly only get 3G, the problem will be far less apparent and you might go for a long time without seeing it.
I’ve tested this and it’s 100% repeatable. Whilst I stick on 3G I have no problem, if I move to an area where I get edge or 2G, when it goes back to 3G there is no data until I power-cycle the ipad.
The ONLY solution at the moment is to use a SIM from a carrier who only use 3G cells. That way you never switch to 2G so never get the problem. Here is the UK, that carrier is Three (www.three.co.uk). I have SIMs from O2, Orange and T-Mobile and all of the suffer the same problem, only the Three SIM is 100% problem free.
If you live outside the UK, you need to research which carrier are 3G only. If there isn’t on in your country, there is no solution until Apple or Globalcomm fix the chip problem. If it turns out to be a hardware only fix, then Apple have a worldwide disaster on their hands.