Twitter has made the API v1 available for an extra month in order to accomodate additional blackout tests before replacing the API with v1.1.
The retirement of v1 was announced in March informing users of migration changes including the removed of legacy widgets aka "Goodie" or "Join the Conversation" and the discontinuation of XML, RSS, and ATOM response formats.
The initial date was scheduled to coincide with the removal of TweetDeck AIR, TweetDeck for Android, and TweetDeck for iPhone on May 7 due to the services running on v1.
Version 1.1 of the Twitter API is the first major update of the API since its launch. The update will bring improved rate limits, JSON support only, updated developer ruls and new Twitter client policies. The overview of v1.1 can be viewed here.
Clients will need to make the move to v1.1 before v1 shuts down as it will cease to work. The extended time will give clients the chance for a smoother transistion between API's.
The date and time of the next blackout test will be posted on @twitterapi soon.