I had the same problem 2 days ago, and it is to do with your PS3.
Reset the PS3, and an automatic message will pop up asking whether you would like the cable to connect, press yes and the settings will be automatically set up and you're ready to go.
If you find that the resolution isn't being used to it's best, then re-visit the settings on the home page of the PS3 and you will be given the option of changing the resolution.
Dundee
