Thewill let you keep both your save data and achievements, and the service is available from December 15 to February 16.
There's a few different variations for this transfer process depending on where and how you purchased your copy of Dark Souls, all of which you can read about. From now on, the Steamworks edition has "become the default version of the game."
A word of warning;, among other sources, is reporting that opting to transfer your multiplayer service over to Steamworks has the unpleasant effect of causing some mods and player-created fixes to fail.
The DSFix mod is probably the most popular way to get Dark Souls looking up to scratch on PC, and it's one of the mods affected by the changeover. Steam user Madot has an in-depthchronicling his attempts to fix the issues here.