Actually, optical does support 5.1, but it has to be a 5.1 encoded source such as a DVD movie or 5.1 encoded game. My creative X-Fi has no problem sending 5.1 over optical from a movie to my logitech z5500s.

It does give the option for optical audio to be used with HDMI, so in essence, there is no real need to be able to support 5.1 on the HDMI cable as the cable hooks up to the TV and TVs don't have 5.1. If you want 5.1 on HDMI 1.3 you still have to run an optical cable from wither your TV or your xbox360, and Microsoft is saying why bother with 1.3 when they can run an optical cable straight from the xbox360. So yes, there still should be a way to get 5.1 and HDMI, just not on one cable.