Along with the announcement that the alpha version has been released, Larian posted a video talking backers through the current state of the game.
According to the accompanying press release, the alpha contains:
• 15-20% of the full game • 10-15 hours of content • Most of the starting city - Cyseal - is available, as well as some dungeons around the city.
Original Sin is set for release in February 2014.