Someone has started a Wiki with details of how you can get pre-paid Data SIMs for different countries, which pretty useful if you want to travel with your 3G iPad.

It still feels a little tricky as a process, but I would think in a year or so you’ll be able to buy one on arrival at a country, and fill it up with your credit card… which would make a lot of sense. Of course the operators won’t like their loss of data roaming charges, but it will have to happen at some point.