jailbreaking ios 4.2.1 baseband 05.15.04

Can this be done yet, I jailbroke my iphone when it was on 4.1 but restored back to normal and then upgraded to 4.2.1 through itunes

I've tried jailbreaking with limerain and redsn0w, it works, but when I click on cydia it doesn't load, the icon flickers

I've seen some jailbreaks which chances the baseband to a 06.15.xx which is irreversible but unlocks the phone

My phone is unlocked through apple, but I just want to jailbreak and not unlock

Many thanks