I haven't heard of anyone dealing with a depleted battery with thePrius. The hybrid system Toyota developed is rather advanced and does a very good job of managing everything so you just drive it and go. If you're looking for more in depth info check out this site: http://prius.ecrostech.com/original/PriusFrames.htm
It sounds like you're mainly interested in the workings of the Power Split Device. That's the planetary gearset that serves the two electric generators, the ICE, and works as a CVT.
As for waiting on an 09, All the talk about extended EV range and improved MPG were counting on the switch to Li-ion batteries, which is now expected to happen sometime AFTER the introduction of the next gen Prius. Toyota is shooting for a 5-10% improvement in economy over the current Prius so it's not like the Prius is suddenly going to be an Insight killer or EV alternative. IMO waiting on the next Prius is similar to waiting no Honda's Insight replacement, or the Chevy Volt. If you can justify the immediate benefit, it doesn't make sense to wait for an indeterminant benefit in the future.