Also, @OP, it's already been confirmed that the Greybeard's will have heavy involvement in the main quest.
I was thinking that when I saw this topic. So yes, I'd say it's safe to assume they'll be involved, and if you're going to have Greybeards in the game, you might as well make them pretty important. Though it's possible that they'll be like Vivec in Tribunal, in that even though the time you actually spend in their presence is not that long, it marks a major point in the story.
I'll say one thing about them, though, I'll be pretty dissappointed if the Greybeards don't actually have beards.