With the full game not due out until March 25th, you're invited to slap down a pre-order right now. If you do, you'll be given the ability to pre-download the expansion files - so when the game launches, you'll be able to start playing straight away.
A Blizzard statement explains that the download will begin automatically once the files are available - so keep an eye out.
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 pre-download, they'll basically get everything that's available whenever they patch up to 2.0.1.
Of course, extra, expansion-only content (like Adventure Mode, Act V and the Crusader class) will not be playable until March 25th - and only then, if you've also picked up Reaper of Souls.
Another major change is coming to the world of Sanctuary - a patch is due out in March to remove the Diablo III auction house, while revamping the game's loot system.