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Before I begin with this tutorial, please note that the tutorial is only meant to work for those having the iPhone 3G currently at a 3.0 or 3.0.1 firmware, we shall attempt to update this to 3.1 using Pwnage Tool while preventing any baseband updates.
Another tutorial for those bearing an iPhone 2G is also on the way, so stay tuned. The following tutorial will only work on a Mac, but we will have something for Windows users out there soon too.
Please backup with iTunes before trying this. Here is how it goes:

1. Grab and download the Pwnage Tool 3.1 for Mac OS X.
2. Make sure to update to the latest iTunes 9 before attempting this unlock.
3. Download the 3.1 Firmware for iPhone 3G and save it on your desktop.
4. Run the Pwnage Tool 3.1 and select iPhone 3G and click next.
5. Select the Firmware 3.1 with build iPhone1,2_3.1_7C144 and click next.
6. Select 'Yes', click next and then select 'No', now the 'Building IPSW' has started.
7. Once completed, select 'Yes' to the prompt and click next.
8. Open iTunes and click Restore (while holding Option key) and select the newly created Custom 3.1 IPSW on desktop.

Voila! the iPhone 3G has now been successfully updated to 3.1 Firmware while preserving the baseband, now follow the steps on this article (starting from step-3) to unlock it with Ultrasnow.
Here is a direct link to download the iPhone 3.1 Firmware for iPhone 3G. In case it breaks, you always know how to look for and download the firmware from your friendly neighborhood torrent sites.
Note: This method will only and only work for the iPhone 3G, not for the iPhone 2G or the 3GS. Also please note that at no point are you supposed to 'Update' to a newer firmware in iTunes, only use 'Restore'. The method we used follows the Simple mode in PwnageTool 3.1.