Out with the old, in with the... hang on!
This may (not) come as a surprise to you, but wearable technology isn't new.
As early as the 15th/16th century (we're not going as far back as a wearable abacus, though) the humble clock was made into a portable form factor: the watch, and it was designed into and worn as a pendant. While this adornment was probably worn more as a fashion item than entirely for practicality, no longer did you have to journey to the nearest room in the house to read the face of a clock on the mantel or wait for the chimes outside to know when it was near the quarter hour.
The need for tracking time is near universal, almost everyone has some reason for knowing the current time and the convenience of having that on your person is hopefully obvious, whether you're an adult needing to know when to finish work or a child making sure that they're home in time for their dinner/tea.
Though early watches were mechanical, as the need for accuracy increased we did not go down the trouser leg of time towards portable Babbage engines on our wrists and end up in the steampunk era, but instead turned to electronics and crystal, and so from quartz clocks, crystal oscillators have helped and continue to be the basis of synchronisation and timings in electronic circuits to this day.
So it should come as no surprise when Google return to the roots of wearable technology and releases the Android based wrist-watch with LG, Motorola. Re-enforcing that as technology advances and becomes smaller, portable and commonly used people want to carry it around with them and the easiest way is to wear it.
So... what is new ?
I feel like I am a child of the electrical computing (or gadget) age, born at a time when the ZX Spectrum was infiltrating homes across the UK and the Game & Watch was sitting in peoples hands. This technology was not wearable, but it was portable and I think it makes sense to reflect that the computers that at one point took up a large amount of space such as a quarter of your typical semi-detached house became small enough to plug into your television at home and take around your friends house.
So in this early era what would be the pinnacle of portability ? For example if you wanted to perform arithmetic or calculus, you would have to rummage through your bag or your pockets, maybe you forgot it and left it at home; whether that's a pencil/pen and paper or a full calculator for example. What if you made sure that you had it on your person as common as wearing a watch? (aha yes it all comes back to the watch) Casio brought out the much coveted calculator watch. These had already appeared in the 1970s but came into mass production in the 1980s. It didn't take much long after that for games to follow.
Now wearable technology is not just limited to a calculator or a game, sensors are being incorporated into devices that we carry with us and wear on our wrists and this concept is being taken seriously. Amazon have a wearable store that you can shop for a device that fits your needs for giving feedback to your or statistically logging information about you. Apple have patented a wearable device to go around your wrist and who could forget, of course, Google Glass. Glass being the other type of wearable which does not just provide feed back, it allows you to capture information about the world around you.
An observation about what makes a technology considered wearable, aside from compact, portable and functional is that it is integral. Wearable technology integrates with what you're doing such that it is not 
obtrusive to your everyday activity. A wrist band, bangle, spectacles, tag, ring, these items are closer to being a part of you and your everyday attire than carrying or lugging around a ZX Spectrum to your friends house. These can be an extension to our memories, storing video and imagery, and senses, informing us about temperature, the atmosphere and weather and it can be the intelligence from some of these devices that really enhances the experience of using them.
It's easy to forget that to wear the technology you also want it to be aesthetically pleasing as well, to not stand out too much or for yourself to feel emotionally comfortable in wearing it. This is where advancements in textiles come into play. Conductive thread can allow for seamless integration of circuitry with little to no disruption in your attire if used correctly. It would also help in reducing the concern when washing the item as well. The conductive thread could be part of a circuit made with say, a Gemma by Adafruit or an ATTiny.
A person attending Leeds Hackspace open evening once showed me light responsive ear-rings that she had designed using LEDs and an ATTiny; it reminded me to work more with ELWire or just to watch Tron Legacy again for inspiration.
Where do we go from here ?
I'm fascinated to see what smart awareness could be built into clothing while maintaining its practicality. Researchers have developed a meta-material which can act as an antenna waveguide in fabric, now I haven't seen a micro-processor directly woven in conductive thread in a piece of clothing - perhaps we won't until we have a recycle-able or safe disposable one (unless it's added as a modular item to clothing, which seems sensible for now).
Any fanatics of Back to the Future (Part 2) will distinctly remember the self drying and adjusting jacket, the Nike shoes (which will be a reality next year and someone kickstarter-ed in 2010), Apple also patented a smart shoe design to tell you when they were at their end of life. So while at the moment a lot of wearables are clipped-on or sit-on our body, perhaps the next stages are to naturally be part of our clothes and become closer to us. You know, before biological and cyborg augmentations come along and we're adjusting ourselves like a modular mobile phone.
What do you think ?