You don't need to be a genius to know that a small wearable computer will have battery life issues at the current state of the art, but Apple has a long history of not releasing a product before it has a reasonable UX, which certainly includes battery life. Four to 5 days would be great but I don't think 2 to 3 days wouldn't be bad.
The UI for a wrist computer might be different than for a handheld iDevice. For example, the back of the watch and inside of the wristband could [be trained to] recognize the wearer's wrist muscle movements to initiate certain functions without ever touching the device. For example the following gesture could...
Feel free to add your own gestures/functions 
2) Is taking off a watch daily something the average person will want to do? That could the affect the user experience to have to fiddle with a clasp twice a day. I suppose those that wear designer watches take it off daily at least to shower and such a small battery might be able to charge within the time it takes to do one's daily ablutions.
3) It can't be as easy to remove as a slap bracelet or it'll be an even larger target for thieves than the iPhone if they can't just grab your wrist as you walk by to steal your wearable computer. I have to assume Apple has thought of this, not to mention that a slap bracelet seems pretty tacky for apparel. Then again, Geordi wore a headband on his face so anything is possible.
A slap bracelet is, by definition, easy to put on and take off.
The unique device ID could be registered by a specific user, and locked if lost or stolen -- it would have little value to thieves.
Just because existing slap bracelets are tacky doesn't mean that an Apple slap bracelet would be tacky or cheap.
I'm not buying one because I don't wear wrist watches anymore. Too bad it can't be self winding or solar powered like some watches are.
If they can get the power requirements low enough couldn't they potentially use a tiny electromagnetic motor that charges the device by the natural motion of the wearer's arm?
I don't wear watches anymore either. However, I have a self-winding Omega (ca 1971) that I dug out, shook for a few seconds, set -- and it keeps perfect time.
I also bought a battery Fairchild LED watch (ca 1973) -- here's a women's version (couldn't find mine or a picture of it):
AIR, this cost $195 in 1973 ($800-$1,000) in 2013 dollars. It weighed a ton! AIR, the battery cost $10-$15 and lasted 6 months. The display was normally off -- you had to push a button to see the time.
Quite a difference between the price/capability of the rumored Apple wristband computer.
Aside: For those who never heard of Fairchild [Semiconductor] -- they are said to be the "founder" of Silicon Valley:
Company history
[edit]1956
In 1956, William Shockley opened Shockley Semiconductor Laboratory as a division of Beckman Instruments in Mountain View, California; his plan was to develop a new type of "4-layer diode" that would work faster and have more uses than then-current transistors. At first he attempted to hire some of his former colleagues from Bell Labs, but none were willing to move to the West Coast or work with Shockley again. Instead he founded the core of the new company with what he considered the best and brightest graduates coming out of American engineering schools.
Only a year later, the staff of eight engineers decided to leave Shockley and form their own company. The group later became known as the traitorous eight. The eight men were Julius Blank, Victor Grinich, Jean Hoerni, Eugene Kleiner, Jay Last, Gordon Moore, Robert Noyce, and Sheldon Roberts. Looking for funding on their own project, they turned to Sherman Fairchild's Fairchild Camera and Instrument, an Eastern U.S. company with considerable military contracts.[1] In 1957 the Fairchild Semiconductor division was started with plans on making silicon transistors at a time when germanium was still the most common material for semiconductor use.
According to Sherman Fairchild, Noyce's impassioned presentation of his vision was the reason Sherman Fairchild had agreed to create the semiconductor division for the traitorous eight. Noyce advocated the use of silicon as substrate — since the material costs would consist of sand and a few fine wires, the major cost would be in the manufacturing process. Noyce also expressed his belief that silicon semiconductors would herald the start of disposable appliances that, due to cheap electronic components, would not be repaired but merely discarded when worn out.[2]
http://en.wikipedia.org/wiki/Fairchild_Semiconductor
By far, the most interesting part [to me] of this rumor is that the wristband computer will run iOS (as opposed to iPad Nano iOS). This means that, as technology allows, Apple can extend the capabilities of this device to make it the only iDevice you always have with you!
– Alan Kay –
– Alan Kay –
