I recently did the exact same thing -- I bought a new 15" PowerBook, retired my flat-panel iMac and kept the Apple Pro speakers because I liked the looks and the sound. I bought a Griffin iFire so I could use the speakers with my PowerBook. The iFire is basically a mini-amplifier (because the Apple Pro speakers need to be powered). It gets the audio signal from your PowerBook's headphone jack and power from your FireWire port.
So, to answer your questions, I see no ill battery effects, but I only use the Apple Pro speakers when my PowerBook is plugged in, so I probably wouldn't see any. To get the iFire to work with the Nano, you'd have to figure out a way to get power to the iFire's FireWire cable.
One more thing -- I don't think that Griffin makes the iFire anymore, but you should be able to buy one from Amazon.com (where I found mine a couple of months ago).