Wired magazine's cover story for their January 2014 issue is all about wearable tech, why it will be as big as the smartphone. This seems inevitable to me.
Smartphones are amazing, and have replaced so many single-purpose devices like cameras, GPS units, and eReaders, but the form factor is not perfect. Output is okay, but you have to pull it out to see it, and input is not even okay; typing on a tiny keyboard is suboptimal at best. (The premise of my new company eyesFinder is that searching with pictures will be way easier.)
Devices which are wearable in some way attempt to address this; displays integrated with glasses for example, to provide "heads up" information at all times, and wrist mounted input devices for easier interaction. These devices are often conceived as peripherals to the smartphone in your pocket, communicating via Bluetooth, and they often gather environmental input, like your location, motion, heart rate, temperature, visual and auditory surroundings, etc.
The article makes a great point: for wearable devices to become mainstream, they must become fashionable as well as functional. I use a Bluetooth headset all the time, but I'm aware of looking like a nerd while using it that bothers me. They are not fashionable. Meanwhile I've been using a Shine activity monitor, and I'd say they are cool.
The main problem so far with the [lack of] acceptance of the Google Glass has not been functionality, it has been that it falls into the "Bluetooth headset" / nerd category of wearable devices, not the "activity monitor" / cool category. Someone will cross this chasm.
We're all iWatching Apple, because it seems like they might be the ones to do it. iGlass? iSee? We'll see :)
The future will be wearable.