Greenpoision RC5, 4.2.1 Unlocking
 

I have read a few posts noting if you install Greenpois0n on 4.2.1 you cannot unlock?

I have a 3GS on 4.2.1 baseband 6.15 (new bootrom) and have just JB'd with GP and it is unlocked to any network, am I correct in thinking this is because I had unlocked it previosly on 4.2.1 with Redsn0w (tethered) ?

you can :)
i just did....

first restore to the custom firmware but before you restore you have to PWNGE the Iphone by Redsnow, should be easy :)
then run GP and after that you will see the emergency screen after that you open redsn0w and click ur 4.2.1 firmware "orginial" then click to ONLY CYDIA and next .....
open ultrasn0w after.
then u are untethered and unlocked