Apparently, Apple has stated that the iPad will never share a data plan with a phone, but user dj6961 stated on the apple message boards that (s)he had already found a way to do exactly that without hacking the ipad. Exact instructions were impossible since the entire message thread would have been deleted, but I think this is worth investigating.

Does anyone know how to access a 3G phone’s data plan using an iPad? If not, who would, and where can I find them?