Namco Bandai announced Dark Souls Prepare to Die Edition at its Global Gamers Day event in Las Vegas earlier today. This PC/Games for Windows Live version of Dark Souls will feature new bosses, new areas, new enemies, new weapons, new NPCs and a PVP online matching system.
For full details, see the official site.
In Japan, Dark Souls was released on the PS3 through From Software. The From Software Dark Souls blog says that an announcement about a Japanese release for the PC version will be coming shortly.