From the looks of it they need to be on the same local subnet, where multicast and UDP work locally. It's almost certainly using UDP for speed's sake.
So for example, the ipad could be on your home airport, and your mac pro could be on the cat5 of that same network, and it would be fine.
Actually it probably works BETTER if your screen mac is on ethernet, because otherwise the two machines are both competing for traffic space on the wireless. In the demo we saw, I bet that alone would have almost eliminated the jitter he was getting.
But since 3G is not on your LAN, I doubt it will work anywhere. It'd probably also be much too slow.
