Jonathan Blow has revealed The Witness will be the name of his next game, and there's even a teaser site to go with the announcement. "Teaser site" is a bit generous of a term, it's a website with some prose that reads like a lost page from Braid's between-level books. Clicking on the text will reveal another page, stating that the "exploration-puzzle game on an uninhabited island" will release in late 2011 on multiple platforms.
Blow had previously mentioned some details about his next title, calling it a "philosophical, and quiet" puzzle-exploration title. As long as The Witness comes with an actual ending that wasn't designed to give academics something to write books about (that nobody will ever read), and leave everyone else scratching their heads, we'll be fine.
Update: Added information about clicking on text.