I doubt we ever see energy efficency like the Palm again. Backlights suck power. WiFi sucks power. 3G sucks power. Java combined with the modern attitude of 'ram and cycles are cheap', which one would have thought would have been revisited by mobile but obviously wasn't, sucks power. People want to watch video. Video sucks power. Typical games on the modern handheld device with 3D, everything animated and playing sounds, GPUs blazing, suck power.
Palm games tended toward static playfields where the CPU spent most of the time sleeping and apps were oriented to 'little black book' sort of data retrieval. When Palm devices moved to color screens and Internet through phone or WiFi the AAA batteries were a distant memory, even without the additional weight of Java.
Barring the introduction of some wonderful game changing battery tech I just can't see battery life getting back to even a week. Any improvement in power consumption is instantly plowed into faster, not put into run time because the #1 complaint is still 'too slugish' and not 'doesn't run all day' because people simply adapted to carrying power cables. And to get to a week you would need to see several of those power drains drasticaly reduced, as no single one would do the trick.
Or we would need a factor of ten or more improvement in battery capacity per unit of volume. With that much extra power available they couldn't just clock everything faster to burn off the power for glitz because the device would actually burn people.