I am the one who uploaded the nb1 rom image. The file that contains the code to lock the phone to ATT is rilgsm.dll, which is included both in the *.nb1 file that I uploaded and in the SP_K1112_230A.nb0 that faramisimo got from Audiovox. My version should be unlocked, but it's an older version of the rom. If you read this whole thread you'll see that I posted what the differences are.
I believe what faramisimo said about rom is correct: there are two parts 32 MB each. One is the (locked) extended rom. This is just a guess from reading the xda forum. To unlock the phone from ATT it should be enough to replace the rilgsm.dll in faramisimo's image with the one one in my image and then repack it. I never got around doing it.

If I understand correctly, a nb1 file is a nb0 file with the directory structure included. I guess that means that the upgrade utility from audiovox to be used with the nb0 file contains the directory structure somehow, and it's probalby harcoded to the specific nb0 file that is provided.

Also, if I remember well, the xda tool don't capture the phone radio rom, which is what you'd need to get your sp230 working again.

Out of curiosity, how did you manage to fry it?