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AppleTV needs a wide-screen display to not have the UI look distorted.

It supports component and HDMI out. If your SD Plasma has either, you should be able to display the AppleTV. If your SD Plasma isn't widescreen, then the AppleTV UI will look distorted.
 
Its a Panasonic 42PWD6 so it's a proper 16:9 widescreen plasma but not HD. I've owned it now for over 5 years and still love the picture quality so haven't upgraded to a HD yet but wasn't sure if the Apple TV will work on it as all the Apple TV specs mention HD etc but never SD?
 
It should work just fine then. The ATV is compatible with HDTV and EDTV's (which do 480p rather than SD 480i).
 
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