domnant,
The 5.1 optical in the back of the television is for the tuner only. It will not perform pass-through operation, such as Blu-Ray-via-HDMI through that optical port.
With your receiver's optical in, you could hook up one of the following:
1. The television tuner (digital)
2. Blu-Ray
3. Game Station
So in this application, it's not possible to hook up the Blu-Ray and Game Station into the TV via HDMI and pass through the optical in the television (it will be transferred to 2.0 audio). So the optical port is designed to send the digital tuner's 5.1 when the channels broadcast in 5.1.
I hope this helps.
--HDTech
just trying to work out if i can get 5.1 sound out of my tv. model name is LA40A450, it has an optical output, but in the manual it seems this can only output 2 channel sound. i have recently bought a 5.1 receiver that has optical in. i want to set this up as some HD programs are now 5.1. can this be done? and if so how do i do it???

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic