Updated : March 20th 23:04 EST
To jailbreak a iPhone 3GS on 3.1.3 you NEED to have your iPhone SHSH backed up, otherwise you wont be able to downgrade firmware and jailbreak, that is the only way. So if you want to restore to 3.1.2 you've had to backed up your SHSH on 3.1.2, get it ? But if you are not on 3.1.3 yet you can upgrade to 3.1.3 using custom jailbroken firmware, see guide below.
To jailbreak and unlock a iPhone 3G on 3.1.3 you NEED the old Boot Loader or you can't downgrade your baseband using fuzzyband and unlock it, but there is no problems jailbreaking it, see guide below.
Πηγή:jailbreakmatrix.com
