Blizzard has discovered a "hardware issue" while conducting internal testing on Diablo 3 patch 2.0.1. As a result, the developer has extended the maintenance period until 8:00pm PST.
"We’re currently working to resolve this issue as quickly as possible, but the issue itself is going to require significant time to address and additional downtime will be needed," Blizzard said. "As a result, the earliest we anticipate servers will be available is 8:00 PM PST."
"We’re really looking forward to players getting a chance to explore and enjoy patch 2.0.1, and are working diligently to bring the game online as soon as possible. We sincerely apologize for the inconvenience and greatly appreciate your patience and understanding. Please stay tuned to this thread for updates!"
In the meantime, you can read up on the many changes that patch 2.0.1 will bring to Diablo 3 once it does go live. A full list of character changes will be made available later.