Diablo III: Reaper of Souls isn't due to come out until March 25th, players should be able to pre-download the expansion some time this month.
On the Diablo III forums, Community Manager Stephany Johnson confirmed that Blizzard is trying to get data to customers in January: "The exact date that the predownload will start is still TBD, but we're targeting this month if all goes well. Note: this download will begin automatically (meaning, you shouldn't need to do anything special)."
Johnson went on to say that if players don't pre-download, they'll get the same data when they patch up to 2.0.1. While Blizzard typically makes some features available once its pre-expansion patches go live, the meat of Reaper of Souls -- the new class, the new act, and adventure mode -- won't be playable until March 25th.