Jeff, you got it, HDMI from cable box into TV for video, then keep the coax/optical from cable box into your AVR, nothing changes for the audio. Then for your Blu Ray, same thing, HDMI for Video, and coax/optical for sound.
You may have to reconfigure your cable box and blu ray player to make sure it thinks it is sending the video to hdmi and the sound to coax/optical, but if I were to guess, I'd bet the devices will figure it out when you put the HDMI in for video and the sound is probably always being sent out of the coax/optical anyway.
There are two rubs to this setup. First, you now need to switch inputs on your tv AND your AVR whenever you want to switch components, this can be done for you with a cheap Harmoney remote if you don't want to change the input with two remotes... and the second one, which perhaps is a bigger concern is the sound quality on blu ray.
The next generation sound, DTS MA and Dolby HD cannot be output over coax/optical, they can only be output over HDMI or analog. So, that is what I was mentioning before, if your blu ray player has analog outs and your AVR has analog in's then you're set! You just stop using the coax/optical for audio on blu ray and switch over to the analog outs, for this you would likely need to configure your blu ray player. If your blu ray player or AVR does not have analog inputs/outputs, then you are stuck listening to plain old Dolby Digital and DTS... which is what you were listening to before you upgraded your tv anyway
