I have the system up on running, complete with low density polyethylene speaker enclosure (aka "Tupperware"). In the next few days I will post photos and schematic, as well as a run down on how to get your raspberry pi up and running without using an attached keyboard or monitor. This is all old-hat for most of you, but for the uninitiated may be useful for future reference.

I spent quite a bit of time hunting around for the simplest possible amplifier that would run off of 5 volts. There are lots of examples of suitable Class A amplifiers out there, but I wanted a Class B amplifier to reduce the quiescent current draw on the Raspi. Most Class B amplifiers seemed to be designed for higher voltages, but I found a nice example of the simplest conceivable low voltage class B amplifier here:

http://www.instructables.com/id/Rechargable-pocket-sized-amplifier/

(The author there further cites "Lupin, introduccion a la electronica", but I am not sure at this moment what exactly that is.)

Class B amplifiers can distort low amplitude components of the audio signal due to "crossover distortion"--i.e, the signal is neith positive enough to push nor negative enough to pull, remembering that transistors do not conduct at all when the base voltage is < 0.7 volts. But in this case, any distortion just adds to the ambience. A class AB amplifier would be better (it uses a bias voltage to ensure that at any given moment on or the other transistor is in the conduction zone), but would nearly double my part count!

I found a very nice tutorial on amplifier basics here:

http://www.electronics-tutorials.ws/amplifier/amp_6.html