[r00fus]

It would a) have to be a first-party cover (preferably by Google) b) have some symbols that indicate 'not recording', c) easy to demonstrate to nearby folks what it's function is (i.e., by removing it and putting it back on).

If Google pushed this and marketed it, it'd really take the edge off the privacy concerns about Glass (I'd still not buy one but I wouldn't necessarily force a wearer to put them away before entering my house).

Relying on your userbase to "not be glass holes" is a recipe for backlash, even against those who "use responsibly".