CrypTweet is an attempt to create a tool to encrypt Twitter DMs. Unfortunately, pretty much as soon as it was released it faced an enormous backlash for a number of implementation errors. The author frankly admits to the numerous problems on the CryptTweet website now and says the tool is only for experimental purposes.
Personally, if I had a need to send encrypted Tweets I’d use one of the PGP tools for smart phones or computers (such as AGP for Android) and encrypt the text before posting.
Some critics of CryptTweet wondered what the point would be of encrypting Twitter DMs. However, I can think of some interesting ways to use something like Twitter in conjunction with PGP to securely coordinate activities that might otherwise be easily observed/intercepted by police and other government authorities.