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I ave the 55 inch version of that Samsung TV and experienced the sam thing, it would play off of USB but not over the network from my DNS32 and would give me the format not supported error. I renamed the .mkv extensions to .mp4 and now all of my .mkv files play over the network. Give it a try and see if it works for you too.
O M G ! It worked! Thank you so very much. You are the one, you are the man! (subtitle .srt files are not supported, too?)
Why don't you try the sMe thing change the .mkv extension and see if that works on the pS3. I think MKV's are just a wrapper.