5.1 surround sound via optical out without receiver
If you want to have 5.1/7.1 (multichannel) audio, you can do it via the optical out (mini-toslink), the HDMI or the Mini-Display Port.

On the receiver side, you must have a decoder, able to decode the stream (PCM for 2 channels and DD/AC3/DTS and their variants) and convert it to analogue signal.

I had the same dilemma when I got my ATV2, I wanted something to be able to utilize my optical SPDIF, and to be able to decode Surround sound. After doing some research, there were several solutions:


Optical->RCA analogue convertors. Stereo only, like GefenTV. This doesn't do multichannel.
A small size 5.1 Decoder - you can read the HD Audio Rush review here.
A Logitech Z-5500.
Full-blown 5.1/7.1 amplifier.


Personally I decided to go with the amplifier and bought a Pioneer 7.1 receiver.