Source from: http://us.battle.net/d3/en/forum/topic/5837135682

We will be performing scheduled maintenance for Diablo III starting tomorrow at 3:00 a.m. PDT in order to deploy patch 1.0.3. We anticipate that maintenance will conclude and that all services will be available by approximately 1:00 p.m. PDT.

Important: Please note that you will not be prompted to download patch 1.0.3 until the patch is live in your home region. If you are logging in from a European or Asian client, you will need to wait for this patch to release in that region before it can be installed. Additionally, if your home region is the Americas, you will be unable to log into Europe or Asia using Global Play after patch 1.0.3 is live until those regions have also patched.

For reference, here are the scheduled maintenance times for each region:

The Americas: 3:00 a.m. PDT (6/19) to 1:00 p.m. PDT (6/19)
Europe: 6:00 p.m. PDT (6/19) to 4:00 a.m. PDT (6/20)
Asia: 1:00 p.m. PDT (6/20) to 11:00 p.m. PDT (6/20)

Please note that these times are subject to change. For time zone assistance, please visit: http://everytimezone.com