Thatcher - October 29, 2012 at 1:30pm
If you purchased your phone from Apple or your carrier and it came unlocked then the firmware won't matter. This is because when you restore your phone during the downgrade process iTunes checks with Apple to see if the phone is unlocked and then grants the unlock remotely. Apple have a master list of all handsets that are unlocked and connecting to itunes is the way the phone confirms this.
Be careful however, if the phone has been unlocked by any other method such as the Dev-Team then you WILL lose your unlock as this is baseband and firmware based and not authorised by Apple.