As I just saw on the Dark Souls Facebook page. "There will be a period by which Games for Windows Live Dark Souls: Prepare to Die Edition users will be able to transfer their save data (characters + progress) and achievements over to their Steam accounts."
If you originally purchased the game through the GFWL marketplace, first, who are you? I didn't know anyone used that thing. Second, you'll be able to transfer your purchase and redeem your game on steam.
I don't have a ton more to add, just that the PC port of one of the greatest games ever made just got slightly less busted, and that's a thing worth celebrating. (For the record, the Dark Souls PC port is totally playable, especially when certain fan-made fixes are downloaded. Also, while I know there are those that disagree with this, I would not want to play it without a controller. Those KB+M controls... Leave something to be desired.)
Edit: Achievements can now be seen in the Steam library. Excitement confirmed.