So I finally decided to downgrade my iPhone 3G from iOS 4.0.1 to iPhone OS 3.1.3. For some reason, Apple does not keep the old ipsw restore files easy to find and most posts on Internet are missing a link to them. Here is a list of some of them:
iPhone 3.1.3 firmware for iPhone 2G
iPhone 3.1.3 firmware for iPhone 3G
iPhone 3.1.3 firmware for iPhone 3GS
I have downgraded my phone following this article: Downgrade iOS 4.1 iPhone 4, 3GS, 3G and iPod touch to iOS 4 / iPhone 3.1.3 / 3.1.2 [How to Guide]
The process is super simple, I couldn’t believe it was so easy, specially, since RecBoot runs so clean and fast. What the tutorials don’t say, although I expected it, is that you will lost all your data (as every time you recover). The problem is the next time you connect to iTunes, you wont be able to restore from your latest backup since it would be for a newer OS version.
Luckily, iTunes keeps track of old backups. I’m currently restoring from a backup I have from April 2010, probably the last one before I updated to the back then beta version of iOS 4.

Downgrading my iPhone: http://dev.enekoalonso.com/2010/08/22/list-of-iphone-ipsw-firmware-files/
This comment was originally posted on Twitter