Well, I got the PS3 and here is my early results. Blue-Ray movies play in 1080i. Games vary so far: Resistance Demo plays at 480p, NBA 07 demo plays at 1080i. GT HD plays at 1080i. Motorstorm plays at 480p. The retail game Full Auto 2 plays at 1080i. There are a couple of games that do state 1080i on the back of the package, so never say never. The only thing I've had to go 480p is Resistance and that was expected.
Remember, my TV only does 480i/480p/1080i. It will not display 720p AT ALL.... and this was via HDMI-DVI cable.... with optical sound.