There IS a way to get it back, but you need a new regular sized never been used before sim and a original 2G iPhone.
Restore the 2G iPhone and then place the sim in it. Once that is done, connect the phone to iTunes and when asked, you are replacing your current phone with the original iPhone. Select your unlimited plan +200 SMS or however many you need and wait as the phone activates.
You now have unlimited data and now place that sim in your 3G or 3GS phone.
If you want to use it in a 4, you need a Micro Sim and a simple phone to AT&T to have the sim activated so that you can now use it in your 4.
Restore the 2G iPhone and then place the sim in it. Once that is done, connect the phone to iTunes and when asked, you are replacing your current phone with the original iPhone. Select your unlimited plan +200 SMS or however many you need and wait as the phone activates.
You now have unlimited data and now place that sim in your 3G or 3GS phone.
If you want to use it in a 4, you need a Micro Sim and a simple phone to AT&T to have the sim activated so that you can now use it in your 4.