Steps

Install iTunes 8.2
Maybe make a backup of your iPhone
Hold Shift and click the Restore button (MAC users: try holding ⌥ while clicking)
Select the iPhone OS 3.0 firmware file
Wait until the firmware is installed (~15min)
- Your iPhone will need activation after bootup, but since your UDID isn't registered at Apple, iTunes will deny the activation
- the trick is to make iTunes think, there's a 2.2.1 firmware on your phone which it will activate without any problems
Jailbreak the phone using QuickPwn (make sure, the Activate* option is unchecked!!)
now use iUtilities or DiskAid:
- iUtilities: Click the String Editor tab and find the section System Version
  - enter 2.2.1 into the first box and 5H11 into the second one
  - click Go
- DiskAid: Open DiskAid, set the starting node to Root in the bottom left
  - navigate to /System/Libraries/Core Services and find the file SystemVersion.plist
  - open this file (or copy to PC and open there) with an editor (e.g. notepad.exe)
  - change the ProductBuildVersion to 5H11 and change the ProductVersion to 2.2.1. Make sure that ReleaseType is Public
  - save the file (or drag from PC back into DiskAid)
reboot your iPhone (hold Home and Power until the slider appears)
launch iTunes and it will activate your iPhone
now you might revert the changes made by iUtilities or DiskAid, see the table below:

Device	Version	Build	Note
3G	2.2.1	5H11
3G	3.0	7A238j	BETA 1
3G	3.0	7A259g	BETA 2
3G	3.0	7A280f	BETA 3
3G	3.0	7A300g	BETA 4
3G	3.0	7A312g	BETA 5
3G	3.0	7A341	Final / Gold Master
3G	3.1	7C97d	BETA 1
3G	3.1	7C106c	BETA 2
3G	3.1	7C116a	BETA 3

SystemVersion.plist

In case you didn't make a backup copy, the file looks like this:

{% highlight xml %}

*[UDID]: Unique Device IDentifier