No, according to AT&T and Apple. In reality, you can swap your iPhone 4 micro-SIM card into the iPad 3G and use your existing iPhone data plan. Jailbreaking is not necessary, only a willingness to change APN settings and violate the terms of your user agreement.
Follow the steps below to use the iPhone 4 data plan on an iPad 3G at your own risk.
1. Turn off cellular data on the iPad by choosing Settings -> Cellular Data -> Cellular Data -> OFF.
2. Take your SIM card out of the iPad and iPhone 4 with a paper clim or SIM removal tool. They're called "micro" for a reason, make sure you don't drop them.
3. Keep the SIM card trays from each device separate, because the iPhone 4 SIM tray won't fit into the iPad. You are only swapping the cards from one tray into the other.
4. Make sure the iPhone 4 SIM card is installed in the iPad SIM tray and slide it into place where it belongs in the side of the iPad.
5. Adjust the APN settings on your iPad by selecting Settings -> Cellular Data -> APN Settings. Put the following in the proper fields:
6. Turn cellular data back on by choosing Settings -> Cellular Data -> Cellular Data -> ON.
Enjoy using your iPhone 4 data plan on the iPad 3G. When you're finished swap the micro-SIM cards back into the proper device.