I think Apple may surprise us again! I really hope the battery life will be great. Battery life is maybe the #1 spec on a smartwatch.
I think Apple have "leaked" these battery times to limit expectations. When Tim says it will last two days everyone in the audience is going to go wild.
Watch has poor battery life because of stories like this one. The story may or may not be true, but I can assure you it wasn't leaked by Apple so they can have a gotcha moment at launch.