Noticed over on Planetdoom that apparently it's been confirmed Doom 3 will not run on Windows 9x:
We went straight to the horses mouth so to speak & asked Activision themselves. The reply we got was that it is confirmed & indeed true that DOOM 3 will only run on Windows machines that have windows 2000 or XP & not lower.
Would you like to know more? No doubt this will a problem for some, but, well, if you're intending to buy Doom 3 you ought to have a new system anyway, so this shouldn't be too surprising, though it is somewhat interesting to see that id is once again at the forefront of getting people to upgrade their systems (Albeit normally graphics cards) ;).