How to Unlock an Apple iPhone the safer way using iPlus 2.0b
Beware of downgrading bootloader to 3.9 using ZiPhone, it is irreversible. You might end with that bootloader forever!! iPlus uses a fakeBL which is reversible! Don't ever restore to 1.0.0 or 1.1.1 though, it will hurt your iPhone for sure!
Visit http://windows-mobiles.com/?p=221 for details!