In the display settings have you turned the PS3 on 480i? If the setting is for 1080p/1080i/720p it might just not display anything.
I have a PS3 which i have been using fine with an HDMI cable,
however I have now had to change to a non HD tv and I cannot get the ps to work with the AV cable (standard-red white and yellow inputs), how do i get the ps3 to work on the normal TV?! many thanks, very frustrating!