This may help you some below. You may or may not be able to use another phones sim in the iPhone.
I have three iPhones, two are 8GB and one is a 16GB. Two are running 1.1.3 and the other is running 1.1.2 On the two that are running 1.1.3, I can switch the sims between the two 1.1.3 iPhone, I just have to connect the iPhone to iTunes for a moment till I see the iPhone is activated message.
If I try to switch the 1.1.3 sim to the 1.1.2 iPhone, nothing happens and the iPhone won't activate untill I put it's own sim back in. I even tried restoring one of the 1.1.3 iPhones with the 1.1.2 sim in it and the iPhone still would not activate. So it appears there is something preventing a sim swap between 1.1.3 and 1.1.2 iPhones but it works between two 1.1.3 iPhones.
I also have a regular cheap Samsung GoPhone that I bought at Radio Shack. It's activated through AT&T with funds on it. SOMETIMES, yes only SOMETIMES, I can take that sim card and place it into my 1.1.2 iPhone and it will work. After awhile, I take out that sim and place the original sim back into that iPhone, after a week or two, that GoPhone sim won't work in that 1.1.2..... until a week or two later. I have no clue why. That sim will also NOT work in either of my 1.1.3 iPhones.
I can just guess that this is something to do with Apple trying to prevent any other sim but the original iPhone sim from working in the iPhone.
Another thing, without an original iPhone sim in the iPhone, the phone won't sync in iTunes. It will prompt you to either activate the iPhone or to insert a valid sim.