Some of the dlc works across all platforms, I know the Lucky Stone does, and I'm almost certain the Blood Dragon Armour does also, I have no idea what the situation is with The Stone Prisoner and Warden's Keep though to be honest.
I was wondering this myself. I purchased the Collector's Edition of the Xbox 360 verison, before info regarding my PCs capability to run the game was revealed. I am now interested in playing it from my PC and would like to know whether to get the standard version of the PC game or the Deluxe Download version (since I already have the physical map and DVD, the non-DLC extras in the DDE version are not useful to me). If my CE dlc items are available across platforms, then I would know which version I need to pick up to get all the items into both versions.
I'm pretty sure if you navigate to Profile> Your registered game promotions then you'll see everything you have for PC XBOX and PS3. For example:
I own the PC Collectors Edition and it says I have available content on both the XBOX and the PS3. HOWEVER, mine is not showing content except for rings and belts. It doesn't show that I have the Stone Prisoner, Wardens Keep, OR content from the PC collectors edition.