When I stick my PS3 on I get 1080i output on the XMB.
When I go to play COD4 is reduces my output to 720p although Assassin's Creed stays at 1080i.
I have no idea why this is happening since COD4 is capable of the higher resolution but I looked at my TV settings and there is no way to manually change the output. I check my PS3's HDMI settings and all is fine there also.
I got my HDMI cable for the equivalent of around $25 so it was a fair deal. The quality is great but I can't really see the need for one. I played my PS3 on my HDTV using a regular A/V cable for a few hours and the only true noticeable difference is the super white setting.