This is just a quick note on a positive experience with Pwnage, used it in upgrading a 1.1.2 IPSF iPhone to 1.1.4 directly. The following sequence of steps worked without problems, although I have no idea whether all the steps I did really mattered.
0) What I have is an IPSF iphone, running tmobile, using the commercial IPSF software, and the IPSF instructions to upgrade to 1.1.2.
1) I downloaded the 1.1.4 ipsw image with iTunes (by choosing the download-only option)
2) I put my iphone into DUF mode
3) using option-click on the restore button in iTunes, I restored the 1.1.4 image (with the precaution that I may want ZiPhone to unlock if Pwnage does not work)
4) restore worked, tmobile was still active, but, of course, the phone was locked
5) Now I used Pwnage according to Pwnage Help instructions -- worked fine
6) Using Pwnage, I built the custom restore ipsw image, without selecting the update of the baseband.
7) following steps 2) and 3), I restored this modified ipsw image.
8) Using iTunes, I synced my computer data, but started a NEW device, i.e., did not restore from backup.
All works perfectly fine.