It would seem to me that if you simply make the change to unlimited with a dumbphone rather than the original iphone, you won't be grandfathered for unlimited data, which is the idea behind using the 2G iphone, no?
Simply taking the iphone 4's SIM and inserting into the dumbphone and adding the new $30 unlimited dumbphone data, waiting for an OLAM update, and then moving the SIM back to the iphone 4 should only work temporarily. But, then again, using the 2G iphone really isn't guaranteed to work indefinitely either, but given that those with 2G iphone contracts were promised unlimited data, it would make more sense to stick to using the 2G iphone.
Any thoughts?