For the millionth time, Diablo 3 is not a single player game. It's an online game that has the ability to play with a party of 1 if you so desire. This is no different to Diablo 2's BNet mode, which was the real way to play it anyway.
P.S. I was surprised at how remarkably few bugs and server issues I had given it was a massive launch. Although some of the lore books went poof when I clicked on them... if a bug has robbed me of lore MY FURY SHALL KNOW NO BOUNDS!!!
---------- Post added 2012-05-15 at 11:47 PM ----------
If it had an offline mode they'd have to quarantine your single player characters like in Diablo 2, which just plain sucks. I'm glad they just got rid of it.