You have to have an active internet connection and a battle.net account to play it, it's as simple as that. There is no way to "get round it".
If anyone tells you otherwise they are lying.
On a lighter note, a rather witty review.
The Escapist : Zero Punctuation : Diablo 3
Hopefully the embed has worked, If not here's the link:
http://www.escapistmagazine.com/videos/view/zero-punctuation/5777-Diablo-3