Alright, let me see if I can clear some of this up.
First off,
1st gen iPhone - the 1st gen is not hot swappable w/ SIM cards. It is activated with ONE sim. If you put a different in, it will tell you invalid SIM or tell you to activate. You have to "hacktivate" to prevent this.
2nd gen iPhone (iPhone 3G) - you can swap SIM cards in the iPhone 3G. Say my friend put him SIM in my iPhone 3G, it would work, he'd get service, no connect to iTunes, etc like the 1st gen. BUT, he would start getting annoying text messages from AT&T telling him he needs the corresponding data plan.
If you do not add this data plan, there have been instances of AT&T charging people PAY PER USE which gets VERY expensive, or adding the data plan anyways, as it IS required, and you would have gotten several texts warning you.
As far as just putting a different phone's SIM in, that will not work. The network recognized the IMEI number of the phone, so if you have your SIM in a Razr, and then put it in an iPhone 3G, eventually, the system would pick up that you are using an iPhone instead of a Razr now. They pick up on this anytime between a few minutes, to a few days.
The only way I know that is fool proof to get away with no, or a MediaNET plan would be to change the IMEI, therefore voiding any warranty.
Just pay for the data plan, the iPhone isn't much of an iPhone without one.
That's my best advice.
Or switch to t-mobile and use the t-zones hack to get cheap data, but you'll be stuck on EDGE.