I have finally had good success with the 3.1 to 3.0 downgrade with AT&T network working.
It is slow and tedious but rewarding.
Please note that the hardest part is having to get the Iphone out of the Itunes recovery mode after updating the 3.0 twice and seeing the 1015 error message twice.
I had to use several computers because I was running Vista and the quickpwn only seems to work under XP and once you run the quickpwn program and hold down the home key for 10 seconds you have to unplug the usb connector and plug it back into a pc or mac running itunes to get the phone to fully initialize.
I did not have to remove the IP address re-direct needed to update to 3.1, since both Apple and Cydia accept the authorization through Saurik's server even with the redirect IP address....The presence of the redirect IP in the Hosts file does not prevent your Iphone 3.1 upgrade (you need to do this first to get the network to be live).
Update first to to 3.1 using iTunes restore... Saurik's server appears to store your authorization request and sends them as needed to Apple's server or blocks them during the jailbrake.
The Itunes redirect IP address can remain in the Hosts file at all times. I was lucky because my Iphone's 3.0 hash files were captured in the Cydia server days before I mistakenlly updated to 3.1....this allowed me to Jailbrake back to 3.0.... Others are no so lucky. It now seems I have the best of both worlds....I have the updated baseband modem firmware 05.11.07 running with 3.0 firmware on my 3gs which is now jailbroken as it was when I accidently did the 3.1 update....Make sure you turn OFF iTunes update message or you may make he same mistake I did.

Bob