I had some confusion finding the correct cable to connect my XBOX360 toslink to a JVC su-dh1 surround sound adaptor. I noticed there had been some other people with the same question, and here is the cheapest answer I could find.
http://www.monoprice.com/products/product.asp?c_id=102&cp_id=10229&cs_id=1022902&p_id=1557&seq=1&format=2
The toslink from the Xbox cable is standard size, the digital input for the su-dh1 will accept the mini-toslink. Much cheaper than finding a toslink to stereo adapter, saves at least $6 by the time shipping is included. I have the Sennheiser 555, and WOW!

http://www.monoprice.com/products/product.asp?c_id=104&cp_id=10423&cs_id=1042301&p_id=2671&seq=1&format=2

This little guy will take care of the issue as well. It lets you use Toslink you already have sitting around, and just adapts it down to 3.5mm. I use to have a bunch of these laying around, as they pretty much always came free with Toslink cables back in the day. The only thing I ever saw that used it was my Minidisc player though. :)