The thing about the batteries that everyone needs to realize, is that there is only so much energy that can be fit into the space provided by the cell(s).
The only thing that can really increase the power per volume is a change in chemistry. Batteries are getting more and more energy dense these days, with many innovations on the way. Some will be able to increase capacity by as much as 10x.
Sometimes the OEM battery makers don't design cells in the most efficient way for the given chemistry, and someone like Seidio can come along and make it better. Or they can tweak the chemistry.
Not TOO much can be done, since major changes in battery chemistry could require different charging methods that the device can't support.
I think the best thing we can do when it comes to a device like the Droid, which can burn through a battery within a day of use, is to plan ahead. Get a car charger, have a spare charger or a cable at the office so you can plug in there. Turn off WiFi and GPS when you're not using them, etc etc...