From the announcement on the Twitter Engineering blog:
Twitter typeahead.js is a fast and battle-tested jQuery plugin for auto completion. Today we’re open sourcing the code on GitHub under the MIT license. By sharing a piece of our infrastructure with the open source community, we hope to evolve typeahead.js further with community input.
Sounds good to me! If you're thinking 'just another jQuery plugin', you might want to take a look at its more advanced configuration options such as custom matching and ranking functions, international language support (including RTL and IME), and client/server mixed search data.
Discuss on Hacker news if you must.