Film is projected with a shutter operating at twice the frame rate or better, to minimize flicker. It would be unwatchable otherwise. See http://en.wikipedia.org/wiki/Movie_projector#Shutter
Well ambient brightness matters, but actually analog cinema projectors use a frame rate of 48 fps (they "flick" every movie frame twice), so it's still way above 30fps.
Plus, with CRTs the problem is exacerbated, because they scan top to bottom, and need higher refresh rate to hide this effect.
A 30FPS screen in similar conditions wouldn't be as bad, but most people use computer screens in well it rooms.
The flickering on an LCD is different, but presents itself as a serious sputtering when you're moving things around.