Quote from at&t:
"Slingbox, which would use large amounts of wireless network capacity, could create congestion and potentially prevent other customers from using the network. The application does not run on our 3G wireless network. Applications like this, which redirect a TV signal to a personal computer, are specifically prohibited under our terms of service. We consider smartphones like the iPhone to be personal computers in that they have the same hardware and software attributes as PCs."
And this differs from a PC with a 3G card how? I don't see any restrictions for laptops, and you are even calling the iphone a personal computer...