If you've tried to hook up an HDMI cable to your Xbox 360 at the same time as a TOSlink optical audio cable, you know about the problem of getting them to fit.  The optical audio out on the 360 lives on the connector of a component cable.  But that adapter is so wide that you can't get the HDMI cable to fit in at the same time.  I came across an excellent tutorial on how to rig up a setup that works:

http://www.chadledford.com/?p=50

This works fine, but it can cause problems when you're switching your HDMI source on your TV away from the Xbox while it's on (as some of the folks in the comments mentioned).  There appears to be a fix for this by disconnecting a wire inside the adapter, but the one that I got was completely hot-glued together.  Solution?  Bought an official MS HDMI kit on ebay for under $15.  Easy way out, sure, but now I can switch inputs without the Xbox resetting.