The reason I sujested it may be to do with the PS3 settings for BD/DVD playback is because in the OP Dan states that gmaes on the PS3 look mint and the Picture issues only occour during BD playback.
Have you tried altering the settings regarding 24htz? and the other HDMI settings like I think its SUPER WHITE or something like that is RGB set to FULL etc?
I have a Problem with My PS3 (been waiting for over a Month now for SONY to come and swap it!) where on the menu screen with video set to 1080p, I get constant flashes of what is best discribed as WHITE NOISE, but boot up a BD Movie or game and the picture is mint. Now according to SONY this is a fault with the inbuilt scaler so maybe You have a simular problem with your PS3 as according to the bloke I have spoken to at SONY, off the record they have a problem with a batch of 40gb PS3's which has led to a shortage in replacement units hence My Month+ wait for a replacement.
SAMSUNG PS-50P96FDX FULL HD PLASMA , YAMAHA RX-V2500 RECIEVER ,DENON DVD3930, SKY HD ,XBOX 360 WITH HD-DVD DRIVE ,PS3 ,Wii ,KEF IQ9'S FRONTS,KEF IQ3'S REAR'S, KEF IQ6C CENTRE ,PHILLIPS PRONTO PRO TSU6000 REMOTE