It took me all nite, but I was able to re-jailbreak my iPhone 3GS after I got stuck in reboot mode after a network reset and was forced to update to 3.1.3. This happened almost 3 weeks ago, and I not only lost my jailbreak, but all of the apps, photos and EVERYTHING that I had on my phone as after I did the update to 3.1.3 I could only set up my phone as a new device. Anyway, I searched online and I found all of the tools I needed to accomplish this task. I downloaded Redsn0w 0.9.2, BlackRa1n RC3, The 3.1.2 ispw firmware, and iRecovery. I also found a post on hacking the drivers folder in the Windows/System32/drivers/etc/hosts file in Windows that would allow 3.1.3 to be downloaded to 3.1.2. It took a couple of hours and some patience, but when all was said and done I was able to downgrade the firmware and re-jailbreak my phone. YEEHAW!!!!!