I use Mountain Lion.
How do I prevent Mail.app (6.0) from marking a message as read automatically, just because I have looked at it in the preview pane?
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An other way to do do (without using the Terminal), is installing TruePreview. It's an 3th party app which allows you to do exactly what you expect.
Do check for a recent version of TruePreview as older versions don't work well with Mountain lion.