As of October 30, 2005:
All iMac G5s, Powermac G5s, Airport Express, Powerbook models 17" 1.67 Ghz models (January 2005 and October 2005), and 15" with 1440 x 960 resolution (October 2005) have built-in optical audio out on their 35 mm ports. These ports can be connected to the
Toslink minijack connector which ships with the AirPort Express Stereo Connection Kit with Monster Cables which will connect to speakers like the Logitech Z-680 to give 5.1 audio out. On the computers themselves, Apple's DVD Player which came with those computers offers built-in 5.1 audio out for those DVDs that were recorded with 5.1 audio.
See also: kimacG5: Optical Audio out and surround sound systems.