In case you need help removing iOS4 from your iPhone 3G, here are the steps that I followed to downgrade my iPhone 3G to OS 3.1.3.  Before we get going, I’m not taking credit for these instructions since this is a cleaned up version of this MacRumors forum post.  Shout out to my buddy Tony Rodgers for sending me the original link.

Required Downloads

Before you get to the steps, be sure to download these two files first.

• iPhone1,2_3.1.3_7E18_Restore.ipsw ( this is the OS 3.1.3 restore file )
• blackra1n ( pick the installer that matches your OS )

Restore iPhone 3G to OS 3.1.3

1. Connect your iPhone, open iTunes, then click your iPhone to go to the Summary screen.
2. While holding down Alt ( OSX ) or Shift ( windows ), click the Restore button.  If you held the correct key when clicking restore, you should get a File Open prompt.
3. Select iPhone1,2_3.1.3_7E18_Restore.ipsw that you downloaded earlier and let iTunes do it’s work.
4. After iTunes tries to verify the update, it should throw an error.  This error is normal, so disregard even though iTunes has left your iPhone 3G in restore mode.  Close iTunes.
5. With your iPhone still connected, open blackra1n and click “Make it rain”.
6. Manually power cycle your iPhone 3G if it doesn’t restart automatically for you.

That’s all there is to it.  I know these steps work because it’s exactly what I did on my iPhone.  The last part of the uninstall is pretty hairy since I wasn’t able to close iTunes without unplugging my phone.  Also, when I ran blackra1n, my phone never rebooted on it’s own.  The screen went black and I just unplugged the iPhone and forced a restart.  When my iPhone booted up, I was back on 3.1.3.

I hope this helps somebody out because iOS4 on an iPhone 3G is horrible.