I'm not that concerned about battery life. It's not related to resolution, it's related to brightness, back lighting, and size of the display. Because of the much smaller display and AMOLED which doesn't have a backlight on all the time, I'm not concerned that it'll be much worse than it is with today's phones.
However, the sheer number of pixels that the CPU/GPU has to push will go up by almost a factor of 3, requiring a LOT more "horsepower" to get the job done. Someone compared it to modern tablets, and that's exactly what worries me. Android tablets (thus far) have not thrilled me with their speed or responsiveness at all. Especially in windows with flash running. I feel like the latest generation of flagship smartphones have finally gotten to the point where they're smooth as glass to operate, but that took a 1.2GHz dual core CPU and it's only at 800x480 res.
I'm definitely eager for that ultra sharp 720p resolution, but to be honest, as I'm accustomed to using an ipad for an ereader, and my current Epic 4G Touch PPI is far better than that anyway, the lower res isn't something I can't live with.