I don't understand much of this technical stuff but wasn't this "DSP"-thing one of the miracle guts of the OMAP Chipset that could optimize stuff alot? ^_^
It makes perfect sense to me.
The CPU uses about 0.05w, and the LCD about 1.6w. Downclocking will knock that down to 0.04w, which doesn't save a whole heck of a lot of battery life.
Craigix always said "about 10 hours", so I don't know why you'd expect 15-20h, especially once you examine the power consumption of the individual parts.
The Pandora as a whole should consume around 2 watts. If the LCD needs 1.6w, then you can shave off a max of 0.4 watts by downclocking and shutting stuff off. More likely, 0.1-0.2 watts. (5-10%) The biggest power savings will obviously come when the LCD can be turned off.
Perhaps if you don't press anything for a minute or two, the OS could automatically shut off or dim it to save juice. (excluding apps like eBook readers) This is the most likely way to boost battery life well beyond 10 hours.
The DSP is a processor that can do lots of the exact same thing. CPUs are optimized for doing things serially. (A, B, C) DSPs and GPUs are optimized to do things in Parallel("A, B, C" is just as fast as "AAAA, BBBB, CCCC", with both executing slower than a CPU could execute the first)
Edit: I read somewhere that the DSP is fixed-point only. That means no decimal numbers. It's good for video decoding, and situations where you can use fake decimal numbers, but I can't think of much else. Since it has a connection to the memory, it's quite possible that the CPU wouldn't have to be woken up to load new data for an MP3 player - if someone can figure out how to code it.
Edit2: My keyboard uses at least 1 watt. It has some super bright LEDs on it, and a tiny LCD. If I used it with my Pandora, I'd expect a severely diminished battery life, assuming it can even draw enough juice from the port.
Also, I have a VIA Eden computer which I use as a NAS. It consumes about 45 watts usually. When it downclocks from 1.2ghz to 400mhz, it consumes.... 45 watts. CPUs really aren't a big part of the picture anymore.