Do some Google searches on this plan. Until recently, some iPhone owners were doing exactly as you propose. Due to that though, T-Mobile recently began "fixing" the the PAYG Sidekick plan.

T-Mobile has at least blocked port 80, and I've read conflicting forum posts regarding what else they may have done--and what they may be doing in the future. Actual Sidekick users have been unaffected by the changes, but most of the iPhone folks appear to be fleeing back to AT&T.

In a few forum posts, there's been talk of slipping past T-Mobile's blockades via proxy servers or VPN connections, but most of the writing appears to be speculative.

There may well be a suitable workaround, but please be aware going in that the PAYG Sidekick plan may not be an "drop the SIM in and go" solution.