As there are many Diablo fans on the forum, but don't want to support a company that wants its customers to singleplayer online aswell, be patient and there is hope. Apparently a Diablo 3 server emulation project has started, and supposedly there are other similar projects aswell to make the game pirateable.
Check these links out:
Not sure if anything of this already really works (haven't tested myself), but feel free to try.
If anyone got more info on this, feel free to share.