Maybe they could link to a phone/tablet or a pebble style watch, alternatively I remember seeing Microsoft have done some cool by projecting a touch interface onto skin. Perhaps the user could do gestures which are picked up by the front camera but it may make everyone look like they're being attacked by invisible bees all the time.

Google Glasses remind me a bit of the time I tried to work out how Tony Stark actually controls all of the functions of his Iron Man armour, and what sort of (realistic) system might come close to what's shown in the movies. I didn't get very far.

To me, Glasses are an awesome concept that begins to fall apart as soon as I think about them - I think they'll either need to pull a major and completely surprising leap forward in how we interact with tech, or at some stage we'll need a keyboard to use them effectively.

eye gestures?