You don't have to pay AT&T twice to access the internet with your iPad 3G if you have an iPhone 4. You can use the iPhone 4 Micro SIM card as long as you introduce the use the settings above.
AU: Only in the United States. -EH
Just put the iPhone 4 Micro SIM card into the iPad 3G. Then go into Preferences, go to Cellular Data and change the APN settings to the ones above. Done. In theory, this should work with any provider as long as you have a Micro SIM and the adequate APN settings. [TUAW]