i had a issue with an jb unlocked 4.2.1 3gs that got accidently reset
when i tried restoring it i still couldnt activate it with redsnow to get to cydia to unlock it so heres what worked for me.
after the cfw 4.2.1 restore if u find yourself stuck at apple logo, put it back into dfu mode run redsnow and only check tethered
then place a att sim card into the tray and when its done and shows the pineapple logo it should kick it to your new home screen, where u can run cydia and unlock it from there for another carrier.
the att sim card is the trick to getting it to the home screen.