As far as I'm aware, yes. Stock 3.1.3 installs a new baseband (05.12.something, i think) and you can never downgrade your baseband even if you can get back to 3.1.2 via backed up SHSH blobs.

Either use PwnageTool to create a custom IPSW for 3.1.2 or if you don't have a mac (like me) then you can download one that someone else made. Just make sure it's for the 3GS, 3.1.2, and either activated if you use an official carrier or unactivated if you hacktivate. Just google "custom IPSW 3.1.2". This will restore you to a jailbroken FW, and from there you can try Spirit again. If you had read the instructions you'd have seen that Spirit is to be run on a clean restore, and not one that has had blackra1n used on it. Good Luck.

Trying to get tethering to work with my phone with no success. iPhone 3GS 32GB with 3.1.2, 04.26.08 modem, older bootrom (week 32 of '09), and running on at&t. Last couple of restores have been with a custom IPSW from pwnagetool installed on a win7 PC, but recently used redsn0w 0.9.3 for windows to change boot logos and enable IPCC hack for tethering. Followed instructions on the wiki and got the mobileconfig installed from the site listed on the wiki for use by at&t customers just fine, rebooted (eventually three times, first with slider, second with SBSettings 'Reboot', then hard reboot) and no change to the settings -> general -> network page to allow tethering. Tried some other mobileconfig profiles I found at other sites, and they either installed and changed nothing or wouldn't install.

Now at&t has not shown in the status bar as my carrier since i upgraded the FW from 3.0, which from what I understood was because of the older modem firmware/baseband that pwnagetool let me keep. Also, under General -> About, "Network" it is listed as Not Available. I assume that this is for the same reason. I would prefer to keep this old baseband simply so in the future I am able to unlock the phone, but I have read that some were having success by upgrading to stock FW (either 3.1.2 or 3.1.3) and then jailbreaking from there and installing the hack.

Advice please?

Trying to get tethering to work with my phone with no success. iPhone 3GS 32GB with 3.1.2, 04.26.08 modem, older bootrom (week 32 of '09), and running on at&t. Last couple of restores have been with a custom IPSW from pwnagetool installed on a win7 PC, but recently used redsn0w 0.9.3 for windows to change boot logos and enable IPCC hack for tethering. Followed instructions on the wiki and got the mobileconfig installed from the site listed on the wiki for use by at&t customers just fine, rebooted (eventually three times, first with slider, second with SBSettings 'Reboot', then hard reboot) and no change to the settings -> general -> network page to allow tethering. Tried some other mobileconfig profiles I found at other sites, and they either installed and changed nothing or wouldn't install.

Now at&t has not shown in the status bar as my carrier since i upgraded the FW from 3.0, which from what I understood was because of the older modem firmware/baseband that pwnagetool let me keep. Also, under General -> About, "Network" it is listed as Not Available. I assume that this is for the same reason. I would prefer to keep this old baseband simply so in the future I am able to unlock the phone, but I have read that some were having success by upgrading to stock FW (either 3.1.2 or 3.1.3) and then jailbreaking from there and installing the hack.

Advice please?