Apparently, in similar fashion to the supposed Millenium Bug, when this counter of twitter tweets reaches a certain number, Twitter-as-we-know-it will destroy itself. Perhaps not as gloriously as that suggests, but it will break.
Of course this could be, like the Millenium Bug, a load of hot air.
The twitpocalypse site explains.
Blogger is being a reet knob'ed and wont let me paste the link. You're going to have to paste this in your browser I'm afraid.