yes it does! period.
No it doesn't. When I say single-player, I mean no dedicated single-player mode. The "single-player" in Diablo 3 is a private game you create on battle.net, it is not a mode you select where the characters are stored on your computer like Diablo 2 has. Therefore no, it doesn't have a single-player. It has solo-play and multi-player. How exactly do you play with a cracked client when you don't even have access to any character data?
From what I read somewhere, being connected all the time isn't merely to "ping" the Blizzard servers. Your characters are saved server side like in WoW and the loot is not shared with your party, it is parsed by the server for your "instance" of the game, regardless if youre in a party or not.
That said, eventually someone will setup a server to emulate the Blizzard server-client setup a-la WoW cracked servers.
Exactly my point, Diablo 3 won't be a game that lets you play offline with a crack that responds to attempted pings to battle.net servers, in Diablo 3, the "single-player" is all server-side, you do not have your character files on your computer, your game client doesn't even have the code to create and run a solo game, the game itself is create on battle.net servers and your client connects to it and gives you feedback as to what your character is doing in your Blizzard-hosted solo game.
And yes, it may be cracked eventually, but not as easily, quickly, or reliably as Assassins Creed 2/Brotherhood/etc.