optimizing and reducing the amount of functionalities might also save a very big amount of battery.
Honestly, what I'd buy, is a
* 4 inch e-ink screen device
* that can close like a laptop, with a physical keyboard,
* with a CPU that sacrifice features and speed for less power consumption
* no camera,
* only wifi, or with 3G disabled by default, no phone function
* browser with a very limited set of feature: limited to css 2, js deactivated by default: it's amazing the amount of firefox modules that can be disabled: http://sourceforge.net/projects/lightfirefox/
* no video acceleration, no graphics acceleration, and no fading/smooth window transitions, 10hz refresh rate is more than enough to read text
I wonder which of those things save the most battery, but I guess all of this would easily double battery life. Also I guess that device would cost much less and be very attractive for students, and be enough to just code if you'd have an online compiler. If you want calls, just buy a candybar phone, like the the latest nokia 100 or 200 series.
I'd buy it even if it cost $300. I wonder how expensive e-ink screen are, but I would not be surprised it's mostly a patent issue.