If the game requires Steam, then you'll need to activate it using Steam, which requires an internet connection. You can, of course, play it in offline mode afterwards, but you'd still need an internet connection to be able to activate it, and there have been single player games that require Steam, like New Vegas, for example.
I refuse to but any games that requires Steam, like New Vegas, for example.
Thus, as of now, we simply don't know the answer to this question. We can only guess based on Bethesda's recent decisions at this time, until Bethesda says whether it will or will not use Steam or any other form of DRM that requires online activation, and that's the only reason I could see the game needing internet access, because of course it's not going to be an MMO. Though considering that Fallout New Vegas used Steam (And while that game was developed by Obsidian, since it was published by Bethesda, I'm sure they're the ones who made the choice.) I wouldn't advise getting too optimistic if Skyrim requiring Steam would stop you from playing it, but we don't know yet of course, so I'm not saying it will certainly use Steam, or any other form of online activation, I'm just saying that I think it's very much a possibility.
And I think the existence of threads like this shows why making Steam mandatory for playing the PC version of the game is a terrible idea.
Agreed, Bethesda used Steamworks for Rouge Warrior and nobody seemed to care, but the game itself was not well received either. Then came Brink with Steamworks, which does include a multiplayer option and Steam, apparently, is a good match for this title. This is most likely why Bethesda felt they should use Steamworks for New Vegas, and that is where the Steam backlash began on the Beth Boards. Did Bethesda listen? Have they learned from their mistake? Do they even acknowledge that it is an issue? Until there is an official announcement we can make no assumptions. When the announcement does come, if Steam is required then I, for one, will stop looking at the Skyrim Threads, if Steam is not required then I’ll join in the conversations about how awesome the game is/will be; either way it is up to Bethesda to make the announcement and thus mitigate the creation of any future Steam threads. I know there are a lot of people tired of seeing them.