All these concepts need a solar element as a battery extender.
My casio pathfinder can go 6 months with no sun after a full solar charge. A smart watch would have a hard time with that, but there is no reason we can't get more than a few days or even weeks on a single charge.
A few ideas floating around my head (besides solar power etc) are:
1.) Motion-activated watch (like those perpetual-motion Rolexes) for increased battery life.
2.) Some kind of hyper-awesome inductive charging. We have our hands down by our sides a lot, so the iPhone could charge the watch this way if the phone is in our front/back pocket.
I feel like there's something going on that Apple is working on - something special. I'm excited to see what it is.