I was looking for a cheap way to get digital audio out of my mac, and it looks like Turtle Beach has a solution. has a post on this litte device. The Audio Advantage is a usb audio device that has analog audio and digital audio out.
With Mac OS X, it looks like it will only output analog stereo audio or PCM digital audio. With Windows, it looks like it will output Dolby Digital or DTS. Also on Windows it can have simulated surround sound.
This looks like a great device for only $30.