I just installed TruePreview 1.4 and found that it didn't work - nothing I did in the Prefs made any difference, messages were always marked as read immediately!

So I removed all other mail bundles temporarily, then TruePreview worked fine. A process of trial and error got me to GPGMail (http://www.sente.ch/software/GPGMail/), when this was installed then TruePreview didn't work.

A bit of lateral thinking lead me to the GPGMail Pref called "Decrypt Messages Automatically", if this is unset then TruePreview works. I assume that GPGMail is marking the messages as read or otherwise interfering with TruePreview.

Running Mail.app version 3.6 (936) on 10.5.8.