In preparation for March's release of the Diablo 3 expansion, Reaper of Souls, Blizzard has announced via "blue post" that pre-downloading of the patch containing all but the exclusive RoS material will begin later this month if all goes well. The patch will exclude the Crusader class and Act V but will feature all of the game-wide changes.
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).
Similar to what we've done with other games, the pre-expansion patch (2.0.1) will contain all of the expansion data that's on the normal disc. So, for players who don't predownload, they'll basically get everything that's available whenever they patch up to 2.0.1.
Caveat: Expansion-only content like the Crusader, Act V, and Adventure Mode won't be playable until March 25th and only players who purchase Reaper of Souls will have access to them.
We'll keep you posted when the download is available.