Ok i am not sure if this had been posted in here...
but recently i needed to verify...
so i got my answer from ude offcial forum....
When a monster with a "?" stat is in your hand, Deck, Graveyard, or
Removed Zone, the "?" stat is undefined. It is not treated as 0. ("Deck
Devastation Virus'" effect is an exception to this)
You cannot retrieve a monster with "?" ATK from your Deck with
"Sangan" because the monster's ATK is undefined. You can get a monster
with "?" DEF if its ATK is 1500 or less. (because you don't care what
its DEF is)
When a monster is face-up on the field, the "?" ATK/DEF value is
usually defined by the monster's effect. How is this is handled varies,
so it is best to refer to the card text and card rulings for the
monster in question.
from here http://entertainment.upperdeck.com/COMMUNITY/forums/thread/1130564.aspx