Twitpocalypse.com defines the event as:

similar to the Y2K bug. The unique identifier associated to each tweet has now exceeded 2,147,483,6471 [...] For some of your favorite third-party Twitter services not designed to handle such a case, the sequence will suddenly turn into negative numbers. At this point, they are very likely to malfunction or crash.

Here's how it may be affecting some of your favorite iPhone Twitter clients:

• Tweetie: Official word: Searches are busted, hoping to update soon.
• Birdhouse: Updated: Publishing is fine, unpublishing is broken. Fix is on its way.
• Twitterrific: Currently experiencing problems after a preventative update wasn't enough to avoid the great doom. Second update in the works per Icon Factory.
• Twitterfon: Unknown, if you have word, let us know.
• Twittelator Pro: Claims to be okay (thanks @JNHohol for the link!).
• Twitkitteh: Is, apparently, unaffected.

If you notice your favorite Twitter client misbehaving, drop a comment below or let us know via @theiphoneblog and we'll update.