Yesterday, I tried to get my old iPhone 3G to work with the GoPhone SIM.
The challenge was to jailbreak the phone, as it's not compatible with jailbreakme.com. After a few trials and errors, I finally succeeded jailbreaking it with redsn0w, although it seems that since I was on iOS 4.2.1, there's now no way to bring it back to "factory settings" as far as the baseband is concerned : I'm stuck with the iPad baseband (6.xx), which is required for the unlocking to work. I of course then unlocked the phone with ultrasn0w.
Had I known that I was going to use my 3G phone instead of my new iPhone 4, I wouldn't have cut the SIM into a MicroSIM. But the damage was done, and so I had to come up with a quick fix for that. I just took an old satellite access card and cut a regular SIM out of it, and then cut the inside of that so that I could fit my GoPhone MicroSIM in it, and voilà, I had my MicroSIM-to-SIM adapter.
I then proceeded to insert them in my iPhone 3G, and the result is encouraging : no "SIM failure" message, just the phone working normally, but showing "Network unavailable" in the top-left.
We'll know tomorrow, when I'm in an AT&T tower range, whether the setup works or not.