I did this on my jailbroken iPhone 4.
- Use ssh or sftp to navigate to /System/Library/Carrier Bundles
- rename the folder ATT_US.bundle
- in said folder there is a file called carrier.plist and in there a section called SupportedSIMs
- in my case there were 7 numbers listed in there: 310150, 310170, 310180, 310380, 310410, 310980, 311180.
- incidentally, those are also files in the folder /System/Library/Carrier Bundles, rename those files as well
- edit the apn settings :-) and enjoy