I believe Giants will be G4 optimized, and ID's mac programmer (gramine?) is working on DOOM3 so his work will probably be merged then.
G4 optimization has always been a game of having to justify optimization for a processor which is already faster than any G3, and thus game companies haven't had any incentive to do it (regardless how easy it may be). Giants is a G4-only game and thus it isn't surprising it's G4 optimized.
I wouldn't be surprised to see more G4 optimized games which 'support' the G3 as a minimum requirement, but 'suggest' a G4.