(adsbygoogle = window.adsbygoogle || []).push({}); " lot of users upgraded their iPhone 3GS firmware from 3.1.2 to 3.1.3 without knowing the consequences. So, if you want to jailbreak and unlock it using blackra1n, you’ll have to downgrade to 3.1.2 but the modem firmware will still be 05.12.01. Remember, before trying this out, back up your data. If you end up bricking your iPhone, I won’t be responsible for it. ( Credit for this guide goes to Cliff who sent it in! )

1. Uninstall iTunes 9.0.3 to from your computer. Remember to back up your data, and rename the iTunes library folder to something else.
2. Download and install iTunes 8.2.1. Download 3.1.2 firmware to your computer. ( links at the end of this post ).
3. Connect your phone to the computer and hold shift and click restore in iTunes. Select the 3.1.2 firmware file.
4. The restore might get stuck and you’ll get an error 1015 message.
5. Ignore it and reboot your iPhone using the power + home buttons.
6. You’ll now see the USB and iTunes logo in the iPhone display. This means that firmware 3.1.2 is installed. But right now, the phone is stuck in a DFU loop.
7. To fix this, you’ll need to use iRecovery. Follow this You must login or register to view this content. at iHackintosh for it.
8. Once you’re out of the DFU loop, iTunes will ask you to sync with the old backup. We suggest you avoid that and opt to setup a new phone. iTunes will reactivate your iPhone.
9. You can now jailbreak/unlock your iPhone using blackra1n RC3. Follow this guide for instructions.

If you are running into issues, follow this guide to jailbreak and unlock your iPhone 3GS.

If you are using iPhone 2G, 3G, iPod Touch 1G or 2G ( non-MC models) follow thisYou must login or register to view this content.."

can anyone verify this method works

It sounds like it will work. I've had to downgrade firmwares before on my old iPod touch and I had to install an earlier version of iTunes and restore, so I guess in theory this will work.

Where's the link for iTunes 8.2.1?