Sure can be done.
Forgive the quick reply, I'm typing this on the PS3 browser.
I have my PS3 connected to my TV via HDMI, but sound is provided via the TOSlink to my 5.1/DTS system.
When it's setup like this, I only get audio through the optical cable, nothing through the TV at all. To get sound through HDMI, I need to go to the PS3 audio setup page, and select HDMI instead of optical out... but yes, you can definately have video only down the HDMI, and audio through Toslink.
V1N.