for the rift, wouldn't you really want 60fps locked? Dropped frames or tearing would be really bad and ruin the immersion.
perhaps games with 'rift' modes could use dynamic details - a bit like wipeout on PS3 can change resolution on the fly to keep the framerate up
Somehow I doubt that it'll be as big a deal for most people than it is for people like Carmack
That is to say, dropped frame rates here and there will be noticeable - more so than normal gaming. And you'll definetly want to make sure that the average frame rate is significantly higher than normal...
But it doesn't seem like a few drops here and there is going to terribly mess up your sense of immersion - any more than sub-par resolution, low res textures, clipping, having your gun locked to head look, etc.
That is, it's a factor that will help immersion, and it's important... but 'not so bad that it'll ruin the immersion'.
Of course if it dips frequently down to 30fps, it won't be nearly as pleasant an experience. Kinda like gaming now dipping down to 15fps.
Sucks, but I can still play the game without been too distracted by it. Of course that varies from person to person, but I think most gamers, if they have to can ignore that kind of thing in favour of getting through the game. Only for a minority does this kind of thing become an intractable issue.