Activision has confirmed that Doom III (that old chestnut) will only run on Windows 2000 and Windows XP, validating rumours that anybody still using Windows 98 would be unable to play the game.
However, as reported shortly after the game went gold, developer id Software is planning Linux and Mac versions in addition to XP and 2000. Speaking last week, CEO Todd Hollenshead said, "Linux binaries will be available very soon after the PC game hits store shelves." The game is due out in the US on August 3rd-5th and Europe on August 13th.
As for the Mac version, "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," Hollenshead said at the time.
An Xbox version of Doom III is due out later this year or possibly early next, depending on how quickly things get done.