Here's a little gem that I was only informed of by my friend Paul Mahon. I had noticed before a little red glow coming from the headphone out on my MacBook, but just figured it was another aesthetic design. It turns out, not only are the mic/headphone sockets on the Mac at line level, but they also send and receive Optical:

Audio line in/optical digital audio in port

Connect your MacBook to a line-level microphone or optical digital audio equipment. The Audio In port accommodates both optical digital audio input and analog audio input.

Analog line and optical digital audio input is accepted through a 3.5mm mini phone jack which does not provide power to a connected device, so you must use self-powered peripherals. The sound input jack accepts line-level stereo signals up to 24-bit stereo 44.1-192kHz sampling rate. It also accepts a stereo miniplug-to-RCA cable adapter for connecting stereo equipment to the computer.

Optical digital audio input is S/PDIF format and uses a standard Toslink cable with a Toslink mini-plug adapter, accepting up to 24-bit stereo and 44.1-96kHz sampling rate.

Headphone/optical digital audio out port

Connect external speakers, headphones, or optical digital audio equipment. The headphone / line output jack accommodates optical digital audio output, analog audio output with a 24-bit, 44.1-192 kHz D/A converter, digital audio output up to 24-bit stereo and 44.1-192 kHz sampling rate and supporting encoded digital audio output (AC3 and DTS). For analog headphone / line output a standard audio cable with 3.5mm metal plug should be used. For digital audio, a standard toslink cable with a toslink mini-plug adapter can be used.

So, not only can you run S/PDIF in from, say, an external pre-amp, you can also run 5.1 optical out. No need to lug an interface around for your multi-channel audio installations!