Can you unlock Iphone 3G 3.1.3

[iJack x]

I recently upgraded to the 4.0 firmware, and found out that it just eats away at your battery life, and is really slow to open things and crashes quite alot.
I jailbroke with redsn0w, and then unlocked my iphone using ultrasn0w.
I've heard that its not possible to unlock the 3.1.3 firmware, id just want to know if any of you have done it?

Thanks