I emailed a couple Larians, and the official word on Steam is that that has yet to be determined (they are looking at several possibilities).
Thanks! It's their decision afterall but I hope that they will not force Steam for D:OS.
Swen himself said this on lar.net:
First of all, the discomfort argument is a very real argument which I experienced several times. Not so long ago I threw a well known game I had just bought through the window, because the online activation didn’t work. It ruined my sunday and I swore I’d never buy a game from these idiots again.
Please Swen, let us opt out of Steam for single player.