Hello. I am another who has considered this, yes.
Some thoughts:
1) Back when the iPhone first came out, I recall reports of people taking their first-gen iPhone SIM and putting it into a 3G device, and getting 3G service. On the other hand,
bacaramac says he tried that and only got EDGE speeds, so I'm not sure what's going on there.
2) Currently, if you transfer an active iPhone SIM between two iPhones, you need to connect the iPhone to iTunes to reactivate it. (Not sign up for a new AT&T plan or anything, just to activate the iPhone.) I would
assume the same process if you swap SIMs from one 3G iPhone to another 3G iPhone. At this point, it's pure speculation on what would happen if you take an active first-gen iPhone SIM and put it into a 3G iPhone. Could be you'd just need to connect to iTunes and activate - or, could be that iTunes has been coded to detect this sort of activity and will refuse to activate the 3G iPhone.
3) Therefore, I plan on waiting until after the 3G iPhone comes out - I suspect that there will be plenty of people trying to use a 3G iPhone on their old $20/mo plan, so I'll just wait to see what they come up with.
