Mac and Linux: Unfortunately I don't have dates for either of these. However, Linux binaries will be available very soon after the PC game hits store shelves. There are no plans for boxed Linux games. More remains to be done for the OSX version of DOOM 3 and that will take some time. We won't release the OSX version until it's just as polished as the PC version. The date for OSX DOOM 3 remains "when it's done", but I can confirm that it's definitely coming.
Demo: our priority was finishing the game first and now we are turning our attention to the demo. I don't have any information to share regarding any specific content, but I don't believe that we have ever disappointed in that regard in the past. We will release the demo as soon as it's done, but this probably won't happen until after the game has arrived on U.S. store shelves.