Video: Dark Souls Prepare to Die Edition (available now for PC and steam):
According to Dark Souls director Hidetaka Miyazaki there will be 3 to 4 new areas within the DLC, 3 to 4 new boss battles, and more NPCs. The equipment of those NPCs will also be available for the player to wear (though if this means you need to murder those NPCs to gain their gear is unknown - knowing DS it probably will, unless of course they are merchant NPCs). Miyazaki also stated that the new area will fit in around the middle of the story, and after Sen's Fortress, and will feature completely new content, though they did have the idea when they were developing the game but it wasn't taken seriously. This leads me to believe it could be something to do with the huge door, blocked by rubble after you defeat the Iron Golem boss at the top of Sen's Fortress - though this is completely sceptical. This would seem like an appropriate point to place something like this, especially as the way you progress to the next area is by air, and the fact that Miyazaki specifically stated it will be 'after Sen's Fortress'.
It has also been confirmed that there will be no ability to create mods on the PC release; unlike, for example, the creation kit in Skyrim. There will also be limited support for the keyboard and mouse, as the PC release is a straight port from the console version, therefore any PC players should probably use a joypad as having precise control over your character in this game is vital.
A full version of the article can be found here: http://www.guardian.co.uk/technology/gamesblog/2012/apr/17/dark-souls-pc-dlc-news
The video below also shows some E3 2012 gameplay coverage for the new edition, including a new boss battle - some of you may recognize this enemy from the original limited edition artwork book.