If several inputs aren't showing any picture at all, call 800-SAMSUNG and set up a service inspection. Since you're under warranty, it's probably best to have a qualified tech take a look.
I have a 32 inch series 5 LCD that is less than a year old. Today when I turned it on I got sound but no picture. I've double checked all the connections and everything seems fine. I even tried playing a DVD and went to the AV source (blind, by memory) and still no picture. I am a direct TV customer but don't see how that would be a factor. Any ideas?