Heres how it works....
Lets say that you start out with the old iPhone before upgrading to the 3G.
Your old sim chip is registered that that old iPhones IMEI #...
As soon as you purchase the iPhone 3G and activate it, your old sim chip is deactivated, and your phone line is registered with the new 3G iPhone IMEI.
Now, when you go to put the new sim back into the old iPhone, you will have to reactivate, because of the new IMEI on your line, and new sim.
Activate the old iPhone with your new sim.
Put your new sim back into the 3G, and it should work fine... Because the iPhone 3G recognizes the sim chip.
I think that what I have gathered with this stuff is true...
Some may be incorrect, but I have truly tried this.