The Blizzard Store has updated with the digital pre-order version of Diablo 3's first expansion, Reaper of Souls. On the store page the release date is listed as March 25, 2014.
The beta for RoS is going on right now, and if you haven't already, you should opt-in to the beta via your Battle.net account page. The impressions of the new expansion have generally been favorable. Keep in mind that while this is a paid expansion, there will be a pre-expansion patch that will deliver many new systems to non-RoS buyers.
Check out our additional coverage of Diablo 3 for more information.
Update: Blizzard is investigating some issues with preordering the expansion. Apparently some live accounts are being affected.