The iPhoneIslam team who previously enabled FaceTime on the iPhone 3G, have discovered a way to use a jailbroken iPad's inbuilt 3G radios for some calling and texting using your SIM card.
The privilege of turning your tablet into a huge phone will set you back twenty bucks. The app is called PhoneIt-iPad and is available for $20 via Cydia.
All it requires is a jailbroken iPad to be an original model running iOS version 4.3.3, and for it to have a normal phone SIM card inside. The iPhoneIslam team note that pre-inserted data-only SIM cards that you may have got from a carrier such as AT&T will (obviously) not work as they are data only. The app doesn't support MMS at this stage, but most other phone features should be enabled, allowing you to turn your iPad into a massive phone.
In the video below you can see both an iPhone 3G and iPhone 4 receiving calls from the app running on an iPad, and also the iPad receiving calls from an iPhone along with carrying out an SMS conversation.
More Info: PhoneIt-iPad