My receiver has an optical audio in port. If I feed it with the optical audio out port on the A2 will I get sound from the center, front, and right channels?
Yes, you will get full 5.1 sound through optical. You will even get 6.1 sound on titles encoded that way.More technical description....
You'll get DTS 5.1 (or 6.1, depending on the title) @ 1.5Mbps.
Every HD-DVD in existance uses at least Dolby Digital Plus @ 640 Kbps, and many titles use Dolby Digital Plus @ 1.5 Mbps or TrueHD @ 1.5-3.0 Mbps. Through optical output on the A2, you will get this audio run through a DTS 1.5 Mbps transcoder. The sound won't be as good as the output through HDMI, but it is still an upgrade over the Dolby Digital 5.1 you get on DVD.
[For comparison, DVD uses Dolby Digital @ 448 Kbps.]