The last Gen iPod Nano is very thin, has great battery life (start recording when you step out, edit later), and when placed in the coin pocket of your jeans the camera lens pokes out just at the top.
DirtyKoala had an ipod belt holster that he modified slightly with a drilled hole to allow the lens to see straight in front of him.
In my experience the pen, sunglass or button cameras are better than nothing but are not great. Tiny pin hole lenses are OK outdoors when the sun is out but suck indoors in low light conditions.