I've been talking about this for years too. But not for security, just to remember everything. Everything. The important thing about this device would be how the video/audio is stored, tagged/indexed and later retrieved. To make this effective, I'd need an easy speech query mechanism, so I can say "Hey when was the last time I changed the oil in my car" or "What did I do with my wallet yesterday when I got home". Or how about "What was the name of that guy I met 8 years ago at that conference and where did he work"?
There are so many tiny events in life that, if remembered, would really help out, so we don't repeat the same mistakes twice. Or just to give "tips" to our future selves, that will always be recalled.