The additional content in the PC version of Dark Souls will be released as DLC for the Xbox 360 and PS3 versions "this winter," IGN reports. The expansion, called "Artorias of the Abyss," will cost $15 through the consoles' respective marketplaces.

There's also good news for those planning to buy the PC "Prepare to Die Edition" in August. Namco Bandai informed IGN that the game will be released not just on Games for Windows Live, but on Steam as well, negating the need for any further petitioning.
[Thanks, Logan!]