No matter what network you're with, global roaming rates for mobile calls are terrible. But nowhere near as bad as the rates for international data roaming. But thanks to the power of the interwebs, the Pay as you go sim with data Wiki is the ultimate resource for smartphone-toting travellers.
The Wiki is a user-generated summary of the different prepaid SIM options around the world that incorporate a decent amount of data in them or are, in fact, data-only. There's information on both regular SIMs and Micro-SIMs for iPad/iPhone 4 users, as well as information on how to get around various obstacles, like carrier-locks and online-only services.
There's currently information for 40 different countries around the world, although the information isn't quite complete - the Vodafone entry in Australia, for example, needs more work. But give it a few months to grow and the Wiki will definitely become a must-have resource in the planning of any overseas trip.