In fact, some e-paper BT display thingies are expected to last few days of regular usage, I've heard (and e-ink offer some nice things re strong ambient light enviromnent). It's worth to mention few semi-open projects (for sure way better, than what "big companies" from first post have to offer):

http://www.metawatch.org/
http://getpebble.com/ (warning, this one uses "e-paper" term in adverts, while in fact, it's not a real e-paper, just very low-power transreflective B&W screen...)

...etc. Sadly, none of them is really open, so people started bundling together a 100% DIY (from base components, microcontrollers, etc) thingies, printing cases for them on 3D printers. That is the really interesting segment of bluetooth watches.

/Estel