How I put my old Cingular SIM in my iPhone and made it work

First I must say all thanks go to the amazing people in the iPhone development team (they don't like links to their site because of bandwidth issues). I especially want to thank "iZsh", "nightwatch", "daeken" and "roxfan" and the rest of the hard working programmers for providing to everyone, the code needed to use other SIMs in the iPhone. I also want to thank a couple of my fellow chatters in the room "Dionisis" for sharing his walkthrough with me when he figured out how to do this and "aPPlewEEd" for catching an obvious error of mine that stumped me somewhere around 2 a.m. and kept me from getting this working.
This procedure does NOT unlock the iPhone after doing this, it still remains carrier dependent. But you can now use most any AT&T (pre-Cingular renaming) or Cingular pre-paid account (mine is from a contract that ran out) or any virtual operator's SIM card that uses the AT&T and Cingular network. This also works for corporate accounts. It has been brought to my attention that if your ICCID number begins with 893103801060, then you have an old style AT&T SIM and this procedure will not work.
This method is for those who live in the United States and can get access onto the AT&T network. Please send all inquiries about service to AT&T. I don't work for AT&T, I can't answer your account questions, nor do I know anything about rate plans or what would work best for you.
Ok, so grab your iPhone, shut it down and remove the AT&T SIM that it came with. Now get the SIM that you want to use in the iPhone and write down the 19 or 20 digit number that is on the card. This number will be referred to later as the ICCID. If your number is illegible, you may be able to determine what this number is by using it in your old phone.
The only other method of determining the ICCID would be to download the iPhone Tool here. Unzip the download. There is a file called tool in it, put it on your desktop. Open the Terminal, type: cd dekstop Then type: ./tool Then, with your iPhone connected to your computer and iTunes closed, type: --activate Now go to the iPhone and press Settings > General > About and scroll down to get your ICCID. Disconnect the cable for the iPhone and close the Terminal. Restart the Terminal and type: cd desktop Then type: ./tool Then, with your iPhone connected to your computer and iTunes closed, type: --deactivate When you get the number, put the SIM into the iPhone.