ok, worked for me first time on my iPad 32gb 4.2.1. already with the tethered redsnow jailbreak installed.
using snow leopard 10.6.1 on my hackbook. downloaded the RC5_2 version of greenpoison.
connected ipad to puter. ran GP (itunes not running). clicked jailbreak on GP, followed the instructions CAREFULLY! got the console blurb, including
Unable to find gBdevList
Unable to find fs_mount
Unable to find fs_unmount
Unable to find fs_load_file
Greenpois0n initialized
i think to bypass this error you have to hold the home button and keep it held on the last instruction in DFU mode. i only let go after the above error and the console resumed its blurb.
after a couple of mins, GP said "complete", iPad reset (apple logo) and went back to the lockscreen. apple boot logo remained unchanged.
no loader icon, irrelevant really as cydia was still present from my old redsnow jb, and all previous settings, apps and cydia stuff on my iPad remains.