After reading Martin35's post i have used also the unlock from these guys. It worked! I have an iphone 3gs 4.2.1. What i like about Iphoneunlockwizard is that they have also unlocks for older firmware and not only the latest 4.2.1. I was able to unlock also my mom's 3gs running on 3.1.3 firmware. Not that she will change carriers, but it's good to know that if she decides to do it, she can, instantly.
The free package is quite good, they threw in a bunch of quite nice ringtones and wallpapers. I haven't tried their apps yet, i'll come back on this one. Well, thanks Martin35, it was worth taking your advice.
I would surely say TheUnlockArena since they gave the codes for my Iphone very quickly and for a cheap price. You can get unlock codes for all GSM mobiles also.