http://www.wowhead.com/guide=cataclysm&guilds

To balance small and big guilds, only the top 20 "contributors" in each guild will attribute their experiences to the guild level every day.
I've seen 10 and 20 as the magic number from various rumormills, but either way it's nice for us. It means everything we do will count and we could even do better than massive guilds full of newbs who mostly just dick around until someone can be convinced to PL them.