Using a Canadian 3G iPad in the UK
This was easier than I imagined. I walked into a 3 mobile store in the UK and bought a pay as you go microsim for 10 pounds. This gives you 1GB of data over 30 days. I popped it in and I had 3G access - no signup via iTunes required or credit card number or anything required.
The only downside is that I have occasionally got a message - no SIM installed. I believe this is due to poor contact on the SIM chip - the 3 microsim is slightly thinner than my Rogers microsim. I might have to put some tape on the non-contact side to make it thicker.