Go into the settings menu on the ps3 and look for display. and video output and change it to HDMI choose automatic detection. make sue that the HDMI out-put is set to the same out-put as the Skybox, both should be ideally set to 1080i for Sky and 1080p for the ps3 if your TV is full HD.
My 42" Samsung Plasma for some reason will not pick up my ps3. It has been connected now for 6 months along with my skyHD, both share the HDMI port (i only have one) they share a HDMI multi connection box. I disconnected the tv sky and ps3 for 2 days whilst I moved every thing around in my front room. On reconnecting, my TV wasnt picking up the ps3. Ive tried resetting my ps3 with the power button(5beeps), connected directly to the HDMI port, swapped all of the leads around (my sky HD box works fine with all cables direct to port or via multi HDMI box).......My ps3 works fine with video cable - Red Yellow White. I just dont understand why it worked fine but now stopped....!!!! Any advice??