My copy from UK didn't require to activate it on STEAM (although I activated it) and you can play it with offline account on GFWL. But I'm playing it with my online account, since multiplayer is the integral part of Dark Souls.
Why is it so integral? Honest question. This came up in another thread and I will absolutely not play this online, so I am curious what the repercussions of that are.
I think that text messages can be helpful sometimes and the world feels less hostile and uninhabited when you know that there are other players you may interact with. And it's nice to get some help with tough enemies. Although getting invaded and killed by other players when you are in human form can be extremely annoying. I'd say you can play it solo, but these online features gives the game more depth.
So now when the game has just been released and there are other players I'll play it online, since now is the best time, when MP is still alive. But I don't mind playing it in offline mode again sometime in the future if the MP will be dead.