Diablo 3 is currently undergoing staggered periods of server downtime as Blizzard rolls out update 1.0.5 today.
Update 1.0.5 adds, among many things, a new Monster Power system. As Blizzard explains: "Monster Power, a new system designed to give players more control over how challenging enemies are in each difficulty setting."
It goes on, "Similar to the 'Players X' command in Diablo II, Monster Power gives Diablo III players the option to increase the health and damage of monsters according to different "power levels" -- and in return, they'll receive scaling bonuses to their adventure stats (including experience, Magic Find, and Gold Find) as well as more drops."
The US servers are down until 3pm today (extended two hours from the original 1pm schedule). Scheduled downtime for Europe and Asia are as follows:
Europe: 6:00 p.m. PDT (10/16) to 4:00 a.m. PDT (10/17)
Asia: 1:00 p.m. PDT (10/17) to 11:00 p.m. PDT (10/17)