[Kruejl]

^^^I totally understand what you are asking, because I did EXACTLY what you want to do. The thing is, you need a "First generation" iPhone right now to do it. If you can find one (ebay), then you can use iLiberty+, WinPwn, or iPlus to activate it and then just put in your old SIM card. When I did mine, I had to unlock it as well to get the phone to find service.

As far as avoiding large data bills, I installed the "T-Zones" hack so my phone can't find AT&T EDGE and I also installed BossPrefs and disabled EDGE completely. That way, there is no possible way the phone can conenct to EDGE. I've been good to go for months this way with ZERO data charges.