If the production is anything like the beta, it's even worse than you think. Yes, you have to be constantly connected and you sign in with your Battlenet account. This has obvious disadvantages, but it has the advantage of carrying your characters and settings around with you when you switch systems. I don't mind this so much.
What really eats me is the fact that not only do you have to be connected to play, your actions in the game are communicated to and from the server. In other words, every time you press a button, there's a transaction with the server, just like in World of Warcraft. So unlike Starcraft where you could connect on any old shitty connection and then play single player just fine, in Diablo III you will lag even in single player if your connection is poor.
It's been a huge problem for me during the beta. It's possible there's something that I don't understand about how to set up my game. Maybe I can avoid this somehow. But if you log in and hit "Start Game" on whatever character while your connection is poor, you'll find the game pretty much unplayable.