Keep an eye on your preferred Twitter client on 11 June, as that's the day when the number of tweets is due to climb above 2,147,483,647. At that point, some third-party Twitter clients may collapse, says the team at Wherecloud, a Canadian mobile solutions company.
The reasoning behind the warning is that the unique identifier associated with every tweet is comprised of a signed integer. 2,147,483,647 is the limit of signed integers, so if your favourite client is badly coded then it won't be able to deal with tweets above that number.
The date is only an estimate, and depends on the Tweets per second between then and now, but you can watch the countdown live at www.twitpocalypse.com - a site set up by Wherecloud - while you stock up your nuclear bunker.