I've since done a bit more research on the batteries used in hybrids, particularly the HCH. Most seem to use stacks of NiMH "D-Cells" rated to 10,000 cycles. That's very, very huge. Since the D-cell size is a standard, that's also a good sign that battery replacement shoudln't cost too much, especially if a lot more hybrids are produced that stick with that as a standard. It's not like laptop batteries which are molded into a separate shape for each model. Literally replacing the D-Cell batteries could probably be done aftermarket for less than $1000.

Also, 10,000 cycles would probably depend on how much city driving you have. If you have a lot of stop and go in your trip, I'd bet the battery does much quicker (a lot more accelerating and decelerating uses the battery more) than someone with predominately highway miles who stays at a steady speed. I would also suspect the Prius would go through the battery quicker than an IMA-type hybrid, as well. I'd suspect most of the cars that have 100,000 in only a few short years have very, very long highway trips. That probably isn't representative of a more typical 100,000 mile car that is 10 years old in terms of battery use cycles.

I actually wouldn't be surprised if upgrades would work simply by supplying a greater amount of amp-hours at the same voltage. From what I understand, the car computer determines the battery level based on the voltage, so having a higher capacity battery could lead to a much more long-lasting assist--one that would be able to remain on much longer for long hill ascents, and such. Such an upgrade might require a ROM update, but there's no good reason that shouldn't be possible.