...I have heard not so great things about the SIM adapters that make smaller SIMs fit in larger phones. If the SIM that comes with the activation kit doesn't fit your phone, I would basically recommend getting the size SIM you need and using it with the activation code from the $30 activation kit, instead of the SIM that came with the activation kit.
Whew! Thanks skimcpip for that additional warning/clarification! Totally makes sense, sort of like when you try to increase a small px photo & make it larger, it turns out blurry. Maybe increasing a Sim-size makes reception "blurry," too.
Bottom Line for 3Gs is to use T-Mo/Walmart/Target SOLUTION.2 in above outline.
What do Sims do anyway? Are they for billing/ID-purposes only, or is the Sim what connects the phone to the wireless "airwaves" in order to get internet access?
Re: Swapping Sims, thanks (blush). I have never messed w/Sims before. I thought Sims were buried under the battery & that to get to it, you had to take off the iphone's back to remove battery to get to the Sim. Great news to learn that is not the case, yes! Thanks!
For anyone else who didn't know: 3Gs Sim hole is on top left next to headphone jack:
http://www.ifixit.com/Guide/Replacing+iPhone+3GS+SIM+Card/1541/1