this is what i've tried:
restore to 4.2.1 in itunes 10 on win7 64bit
run greenpois0n in admin mode
hold down power + home
release power after 10 seconds during black screen
GP says it's in dfu mode
select jailbreak
white screen
connect to itunes screen
lots of disconnecting and reconnecting
GP says "jailbreak complete" without displaying any code on the screen!
Reboot - Not jailbroken.

So I restore and try the process with limera1n
same thing, doesn't work.

so i go to a 32bit winxp machine and try both
Same thing, jailbreak acts like it's complete but fails.

What in the world is goin on here? Why Can't I jailbreak this ipad?

after doing some reading i'm thinking that jailbreaking 4.2.1 is impossible until the new unteathered is released? Is that right? Eg. Limerain and greepoison only support 3.2.2. However all the guides all over the net say 4.1, but don't specify that it's different for the ipad.

redsnow is your friend, search youtube for redsnow jailbreak 4.2.1