I've always been slightly annoyed that Mail doesn't have a keyboard shortcut (or menu item) to toggle showing/hiding the message preview pane, so I wrote a little script to do this. Place the script in your account's Library -> Scripts -> Mail Scripts directory and it will show up in Mail's script menu. To assign it the keyboard shortcut Command-Control-P, I named the script file "PreviewPane Toggle___ctl-cmd-p.scpt". (One can also use the method described in this 10.3 hint.)
tell application "Mail"
tell message viewer 0
if preview pane is visible then
set preview pane is visible to false
else
set preview pane is visible to true
end if
end tell
end tell
[robg adds: To me, one of the neater aspects of this hint is the tidbit on how to add the keyboard shortcut to a Mail script -- I had no idea it was possible by simply naming the file in a certain manner! I've now added keyboard shortcuts via file naming to the two other Mail scripts I use all the time ... very handy! I also tried with a Finder script, but it didn't seem to work there.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003122400410896