by creating a custom firmware by unpacking 3.0 or 3.0.1 firmware and add the file from 3.1 that iTune send for sign from our pc that recognized fimware ver and put it in 3.0.1 or 3.0 by creating custom firmware to trick the apple server think that it 3.1 if it is possible (i think it is possible to do that as in the firmware itune is sending something to apple server and it knows which firmware ver you have and resend the file by signing it to you to go ahead) it is the matter of just one file or bit which send firmware detail to apple

hope this will do the trick for downgrading

Have you read Saurik's article on how to restore 3.0? It explains why this wouldn't work.