Perhaps one big reason that voice commands for computers have not taken off is the corporate market. It's too weird to think of an office full of users chatting to their computers in an environment that already has too much openness to background noise. In a typical office, you have the air conditioning sometimes emitting odd noises, clicks from keyboards and mice, conversations in the kitchen, and people on the phone. To add to that everyone speaking aloud at once would surely drive me insane. So it's not that we can't use voice to drive our machines - in fact, such abilities have long been well within our reach - it's that we don't want to.
Mokalus of Borg
PS - Or at least we don't want it at work.
PPS - It might be okay at home.