This isn't PS3's limitation.
SPDIF, being it coax or optical, can't carry anything else. Unlike HDMI that can carry multichannel LPCM.

You are correct that voice/dialog is most of the time in the center channel and heard in 2-channel mode only if properly folded into them.

Do you go PS3->TV->receiver?
If yes, that is most likely your problem.
Most of the TV's can't pass through multichannel audio, only stereo.

Try PS3->receiver->TV instead.
If the receiver doesn't support HDMI repeating with HDCP (for Blu-ray), run the video directly to the TV and audio over SPDIF directly to the receiver.
You'll lose lossless audio support but it will be multichannel.

Diogen.

EDIT:
BTW, why do you need a splitter? Your receiver has 3 HDMI-ins and 1 HDMI-out.
http://www.sonystyle.ca/commerce/ser...ductId=1005449
And you might want to read from here
http://www.avsforum.com/avs-vb/showt...8#post16054858