CPU is rather a small power of total platform power consumption these days. Switching x86 to ARM won't magically get you from 5 to 50 hours of active usage. If you compare MacBook Air and the new iPad, you'll see that the difference is actually rather small:

MacBook Air 11.6 - 35 WHr battery and 5 hours of Wifi Usage at 7 W [1] New iPad - 42.5 WHr battery and 10 hours of Wifi Usage at 4.25 W [2]

e-ink screen may make that last far longer... but if you've spent any time trying to use the e-ink Kindle browser, you'll quickly realize that the lack of fast refresh rate is a very difficult problem to solve in terms of usable UI.

[1] http://www.apple.com/macbookair/specs.html [2] http://www.apple.com/ipad/specs/

What kind of UI would it have? For interactive use (ie: not just reading), an e-ink UI seems hard.

Newer e-ink type screens are better than the older ones about reducing the "flashing" between screen draws (to avoid ghosting) but they still need to do it every once in a while (I believe new Kindle does it every 6 page turns).

Since I got my transformer (nominal 16 hours battery) I've stopped using my x86 netbook. In theory I can run more programs on the x86, but having that much battery makes a huge amount of difference (I took it on a train across russia recently), and I've been able to find android programs to accomplish everything I actually want to do with it.