To easily upgrade your baseband to 06.15.00, re-jailbreak your iPhone with redsn0w (iOS 4.xx) while selecting Install iPad Baseband. There is a chance to lose GPS though. After that, you can unlock it with ultrasn0w.
Did you try to upgrade to 6.15? If so, did you lose GPS? I am really tempted to try this out, but the main reason I want to unlock it is to use the gps while I am over in Europe, so will kind of defeat the purpose if it definitely breaks it.