Can I use my iPhone 4 data plan on the iPad 3G?

Officially the answer is no. Swapping micro-SIM cards from the iPhone 4 to the iPad 3G will violate your AT&T contract and terms of use, etc.

That being said, it is possible to use your iPhone 4 data plan on an iPad 3G without jailbreaking. Follow these steps at your own risk:

1. Make sure 3G cellular data is off on the iPad. To turn this off go to Settings -> Cellular Data -> Cellular Data -> OFF

2. Remove the SIM card trays from each device using a SIM removal tool or a paper clip. The micro-SIM cards and trays are small so handle with care.

3. DO NOT mix up the SIM card trays, the iPad SIM card tray will not go into the iPhone 4. Only remove the cards from the trays and swap them.

4. Replace the iPad SIM card tray with the iPhone 4 SIM card installed.

5. Change your APN settings on the iPad. Navigate to Settings -> Cellular Data -> APN Settings and enter the following information:

APN: wap.cingular
Username: WAP@CINGULARGPRS.COM
Password: CINGULAR1

6. Turn on cellular data by selecting Settings -> Cellular Data -> Cellular Data -> ON

Now your iPad is using your iPhone 4 data plan and your iPhone 4 is temporarily an iPod.