Default Proxy SIM Cards?

Does anyone have any experience with those "proxy" SIM cards that sit in between the actual SIM card and the phone? Since it doesn't seem like any of us are going to be getting our AT&T sanctioned unlocks for another 10 months, these cards seemed to me like a possible alternative.

If anyone has one, I'd be interested in hearing if you can get it to work with your Captivate.