@Ian2: Amazing. As a regular CAD user, I cannot wait to see what this can do.
This said, I'm kind of worried about how well this would work for the general public, looks like there's going to be a "palm rejection" problem on steroids here. Either users get used to move their hands off the sensing area, or the UI needs to be very picky (or very smart) about intentional and valid gestures. Either way I expect it will need some training.
With a little luck, the coolness of "casting spells" to your PC will be a big enough incentive.
As for the audio commands and feedback, the technology never worked too well for PCs; one thing is to bark commands to an xbox in your living room, another is to have to say what you are doing in public. I doubt that's ever going to change, until they nail subvocalization.
Once that happens, we'll have proven Clarke's third law again: "any sufficiently advanced technology is indistinguishable from magic".