Let's say it actually happened...you wouldn't use just one screen, you'd use 2 HD TV screens. Two screens would get split 7 ways.
Maybe more than 2...Sony have announced that you will be able to connect 2, 1080i (I think it's 1080i) HD screens to the PS3 and have it like a Panarama view, but they haven't said whether you can have more than the 2. Still a long time until it finally comes out...plenty of tinkering time. It's a long shot, but one of the clever guys developing the PS3 may think "7 controllers...if they play multiplayer maybe they'd each want their own TV?". Probably not 7 screens when we finally get it...but I'm guessing more than 2.
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