The technology is cheap; key fob cameras are light and cheap, and can be carried easily. Here's footage shot with a small camera (< 1 ounce) mounted on a cheap "foamie":
http://www.youtube.com/watch?v=u9_jwJ68IWI
This is several years old; the same cameras now do HD and/or shoot high-resolution stills at selectable intervals.
Like Wes said, the main issue is legal....
Ron Wanttaja