OK, if you are stuck with a 3GS and need to downgrade (just remember, it won't downgrade the 3.1 baseband) or need to restore but don't want to go to 3.1, perhaps this will be of help.
It was just posted a few minutes ago....
TinyTSS
Apparently the "Umbrella" portion collects the files needed to sign any prior IPSW file, then stores the files on your computer.
Although I don't have a 3GS and don't have this issue, I wanted to pass the info on.
I'm sure your phone needs to be operational, not stuck, to collect these.
It was just posted a few minutes ago....
TinyTSS
Apparently the "Umbrella" portion collects the files needed to sign any prior IPSW file, then stores the files on your computer.
Although I don't have a 3GS and don't have this issue, I wanted to pass the info on.
I'm sure your phone needs to be operational, not stuck, to collect these.