I previously used the information contained in this older hint to view mail messages stored in Microsoft's TNEF or winmail.dat format. The Command-[ keystroke provided by the past hint no longer works in Leopard.
Not to worry, Mail can still display the text contents of these messages with the keystroke Command-Option-P, which in the menu, is the View » Message » Plain Text Alternative item.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007112012461959