Restoring and unlocking iphone 3g has been changed due to itunes update
steps are pretty simple
1- simple Restore Your iphone using Original firmware
4.2.1
after restore if you have 6.15 baseband Phone will be stuck on Recovery mode ( use tinyumbrella or ireb or Redsn0w to kick it from Recovery and phone should work ok
2- jailbreak your Phone using
Redsn0w
You can Skip kicking your Phone from recovery and jailbreak and phone should be ok
3- install ultrasn0w from cydia (check guide in this thread )