Years ago I had some Tritton AX360 headphones (I wanted true surround; the result was mediocre), and they came with a terribly interesting little device... This little sucker can take SPDIF or digital coax input, and it has 3.5mm 5.1 jacks on the side for use with PC speakers. This device is meant solely to complement the headphones it is bundled with (hence the proprietary headphone jacks on the front). Since it can handle Dolby Digital, I had used it in the past to play 5.1 game consoles over my PC speakers. Bottom line is, I have some old 5.1 PC speakers I'd not yet depart with and a device like this would extend their usefulness significantly. Essentially it turns old analog speakers into digital ones. I know there is a thread over in Console Gaming where this type of device would answer the OPs prayers.
You mean something like this little bugger http://www.rackmount-devices.com/main.html?page=012-0154 Not the greatest sound output, but acts as a analog 5.1 bridge via USB. However it is meant more for PC and not console usage. But to answer your question, yes there are devices that can do what you want
