It was hinted before that the Open Beta would go live sometime December, but now it's official - Open beta will start next year, January 23, 2013 US time. A bit of a delay, but from what I gathered from other players of the game, a one month delay isn't so bad.
This move to a January release is to prepare for the upcoming issues they might encounter during the Open Beta. Since it's now the holidays, Grinding Gears won't be able to give the Open Beta the support it deserves since some of the developers will be on holiday break. Moving it to next month gives them time to rest up and prepare for the Open Beta.
Path of Exile game systems explanations
Together with the game heading to Open Beta, the game's third act will be available to players. The 0.10.0 patch will be implemented and will include a final character wipe when the Open Beta kicks in.
Path of Exile has been in Closed beta since August 2011.
[Source: Grinding Gear Forums]