A blue posted about how the Dark Phoenix and other guild vendor items will be locked until your guild gets a certain achievement.
He said "Let's just say, that for example, you need to complete the new guild achievement 'We are Legendary' in order to unlock the Dark Phoenix. That achievement requires the guild to gain access to all 6 legendary weapons currently available in the game."
He went on to say you'd need a certain guild reputation to then buy the item.
Please note, and I'll put this in bold to emphasize: he gave this as an example. We do not yet know what achievement will unlock the Dark Phoenix. I like that it has to be unlocked, though I do hope it's not connected to Legendary.
Another item of note that I think everyone should pay attention to:
(note that all guild achievements start on Cataclysm launch, so anything you have now will not matter, it must be done with your guild after launch)