no, you're screwed if you already updated to iOS4 and never jailbroken before, for now
Are you sure?
I did the update to 4.0 on the release day. Then this evening I followed the instructions here
http://www.addictivetips.com/mobile/downgrade-iphone-3g-3gs-from-ios-4-to-3-1-2/
and managed to put 3.1.2 back and jail broke than and used the Pwnage tool from there.
In the link above ignore the first instructions (They don't work). Go down until you read a comment about adding a line to /etc/hosts
I'll post that bit here below. It mentions the path for windows but the host file is the same on the mac just that it's /etc/hosts
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1. Open the file C:\Windows\System32\drivers\etc\hosts (Windows) or /etc/hosts (Mac OS X) and add the following entry to the bottom of the file.
74.208.10.249 gs.apple.com
Restore
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2. Connect your iPhone to the USB and put it into DFU mode. iTunes will display an alert stating you need to restore your device, click ok then hold option + click restore and navigate to your previously downloaded 3.1.2 FW.
3. After the restore finishes you should be hit with a 1015 error. Thats expected. Your phone should be in Recovery mode now and iTunes will tell you to restore again. Click OK then hold option + click on restore again this time you do not have to go into DFU mode and navigate to the 3.1.2 FW.
Jailbreak
4. After the restore finishes again itll throw another 1015 error. Thats expected again. This time, just select OK and close iTunes. Fire up redsn0w make sure its the 0.9.4 version and jailbreak as normal. Uncheck all the ticks while jailbreaking.
5. After the jailbreak is finalized your device should reboot and strange enough you successfully downgraded to 3.1.2.
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