It really isn't a catch. The folks at iPhoneIsLam have been working on an application that will turn your iPad 3G into a fully iPhone-capable iPad. The way the iPad 3G was meant to be in the first place.

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This means the result is that iPad 3G users will be able to make and answer calls, send texts / SMS and FaceTime � no hardware alteration whatsoever is needed.

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The application which is still under development has been featured in the video below, as you can imagine, this sort of 'hack' won't even make it to the front door of the App Store. So PhoneItiPad will be headed for Cydia which means iPad users will need to have their device jailbroken first, but otherwise there really isn't any catch.
[via GadgetVenue]