If you are attempting to log into Diablo 3 this morning, you are likely running into Error 3003: "Unable to connect to the service or the connection was interrupted."
This is because Blizzard is performing scheduled maintenance and is applying patch 1.0.2.
Today's patch is "mainly aimed at addressing service issues", Blizzard confirmed. The first "real game balance changes", outside of hotfixes, will be coming in patch 1.0.3.
The maintenance began on Tuesday, May 29 at 3:00 a.m. PDT and is expected to be available again at approximately 11:00 a.m. PDT. As with most patches, especially early ones, expect some technical difficulties and extended maintenance (hopefully this doesn't happen).
During this time, Diablo 3 is unavailable. Additionally, the Account Management, Blizzard Store, Forums, and website will be unavailable until approximately 11:00 a.m. PDT. We will be sure to update you once this patch goes live.