I have a 5.1 sound system that only has just the one digital optical input
I was wondering if the kdl40w4000 tv can output a 5.1 from its optical connection if the 5.1 sound is inputed via a ps3 using the hdmi conection.

I would say not, don't know any TV that has an optical output than can pass DD5.1 from another source.

You could get an optical switchbox, then you could connect the PS3 and TV via optical. You would still get DD5.1/DTS from the PS3 but in addition you should be able to listen to the TV via the sound system, then depending on the system you might be able to apply DPLII/DPLIIx processing giving you something near DD5.1 audio, if this is not offered you should have 2 channel stereo, in most cases this tends to sound better than using the TV's audio.

optical switchbox sounds to me the way to go, just want to keep things simple
thanks for your help