Using different Carriers??

I have a Prepaid AT&T account right now that I am using in my Inspire, I have unlimited talk and text, no data.. I use wifi when it is available. I would like to take full advantage of my phone and use the data without a contract.

Does anyone have experience with the $45 Straight Talk? I hear they make phones that are AT&T based and that they can be used in other smartphones thanks to their SIM cards (some Straight Talk phones are Verizon based). Im just trying to find out how easy it is to convert it to a new phone using the provided SIM.

Right now the only phones I am seeing that have the proper SIM to be used in other phones are the AT&T Nokia E71 and Nokia 6790.. Obviously since I will not be using either of the phones, so will be stuck paying $200 for a SIM card. Are there cheaper phones available with the proper SIM cards?

I understand that there are risks to doing it. With ST you are able to use your existing phone number. If they happen to cancel an account with that phone number, is it possible for you to switch back to your previous carrier with that same number or does the phone number have to be registered with an account to be brought up in the new carriers system? Is there a way to tell which phones have been patched and which have not?

I see that some phones have been cut off for making international calls without having the service, calling customer service from the hacked phone instead of the ones provided, and being stupid with the streaming and tethering. Checking the serial numbers on a phone that has been purchased, sold to a different user and registered under a second name is the only way I can see ST knowing that you have that SIM in a different phone. If I am planning on keeping the phone that the SIM came with is it still possible that they will be able to track its use to the new phone?