I was pretty sure this would be possible, but reports about this working out there are sketchy at best so I wanted to put it to the test. Since I now have a test iPhone to break before selling it on, I upgraded it to 1.1.1 to dump the filesystem over the past few days. I also got to test to see if the upgraded modem baseband firmware would still work with a TurboSIM. To test, I downgraded from 1.1.1 to 1.02 - the modem firmware obviously remains intact from 1.1.1. Everything works as before with baseband version 03.14.08_G. I was half expecting to maybe run into some issues with Applesaft, as 04.01.13_G reads the AT&T IMSI three times as opposed to one, but all seems fine.