I'm not sure that I'd feel comfortable carrying a fan rotating at 20,000 revolutions per second in my cellphone, which I keep in my pocket, which is next to my...
The thing produces 10 watts. I don't know if I'm just being paranoid about wondering how it might fail catastrophically (I don't fret about batteries blowing up, even though it's obviously possible). I suppose you wrap the thing in titanium and have "crush zones" like a car.
Update: Added link so you know what I'm talking about.