For anyone who have accidentally upgraded to iPhone OS 3.1 and would like to downgrade back to OS 3.0, you can follow this guide for the downgrade. This guide is written for iPhone 2G and 3G only. It will not work for iPhone 3GS.
By downgrading your iPhone 2G/3G from OS 3.1 to 3.0, you can use redsn0w to jailbreak it again. But for iPhone 3G user, while you can jailbreak your iPhone, ultrasn0w will not work for you as iPhone OS 3.1 has upgraded the baseband.
Okay, let’s get started.