Apparently the Optical Out supports surround sound formats, but then of course Dolby Digital and DTS are simply matrixed stereo tracks, not 6 discrete channels. I don't suppose Apple will be realtime software Dolby encoding into MacOSX 10.3?
Also, the optical in supports externally clocked sources up to 96KHz, but internally only up to 48KHz, which is the ceiling on the optical out too. Bit weird. I was kinda hoping Apple would duplicate what the SGI Octanes have had for years (ie. a pair of TOSLINKs supporting ADAT and S/PDIF.