Well, Luffy's only in the first half of the Grand Line, isn't he? And Whitebeard and Shanks are in the 'New World'.
Now, there exists the possibility that Shanks or Whitebeard could get to the One Piece before the Straw Hats.
Of course, there is also the theory I heard the other day about Raftel being underwater, and Shanks and Whitebeard are just waiting for the tide to recede.
Opinions?