I have just moved my PS3 to my PC monitor so that we can watch tv too. I have the HDMI cable now going into my monitor via a DVI adaptor.
Whenever I switch to the PS3 it states Its not optimal mode recommended mode : 1680x1050 60Hz

I know the 20" Samsung T200 wont handle 1080i/p and have change the HDMI cable aoutput settings to 576 and 720 but it still pops up and after a few minutes turns the ps3 picture off and leaves the message bouncing round the screen.