Now I know Blizzard has not yet said the release day, but the company in my country has always been spot on for release days so i trust them and if you look at the link about half way down the page you will see Diablo 3 list as the 1st of May to be released.
They usually dont put the date up unless they know as you should see at the bottom of the page so hopefully this is correct
http://www.btgames.co.za/bt/release.asp