After switching to mail.app from Thunderbird, the blogger at 48-Hour Days found that that she (or he) couldn’t live without Thunderbird’s F8 keyboard shortcut for showing and hiding the Preview Pane.
As everyone knows, you can show or hide it in Apple Mail by double-clicking the small dot in the separator between the Mail Viewer and the Preview Pane — Apple’s Technote 
shows you how.
But if that is not quick enough for you, 48-Hour Days provides an applescript that will automate the process and which can be bound to a Quicksilver trigger or FastScript’s shortcut.
Only hardcore keyboard fanatics will want to use this, but there are people like that out there, and this post is for them.
Of course, binding the F8 key may not be the best key, since it activates Spaces’ preview screen in Leopard… Perhaps Ctrl+F8 or Command+F8 or something else would be better…
You can also simply use the Shortcuts List in Keyboard and Mouse System Preferences Pane to assign a keyboard shortcut to the menu item in Mail. That way you don’t have to worry about this shortcut disappearing when Quicksilver crashes, and it can be a bit snappier at times than waiting for the AppleScript engine to kick in.
@lar3ry: Absolutely, if you use Spaces, then binding this to the F8 key won’t be the best choice. I never got into Spaces, so for right now, F8 is fine for me.
@David: I didn’t think there *was* a show/hide preview pane menu item? I can’t find it in Leopard’s Mail.app – where’s it hiding? For people with vision impairments like mine it’s very hard to be exacting enough with the mouse to click on the tiny dot and show/hide the pane – I would prefer a regular keyboard shortcut sans AppleScript if it’s possible though.
r
The preview pane “show/hide” is under View->Preview Pane->Show/Hide. If you use the keyboard shortcut method in the system preferences, go to the keyboard & mouse preferences pane, click the ‘+’ sign to create a new shortcut. Select Mail from the Applications drop-down menu, type ‘Hide’ for the menu item and choose your keyboard shortcut. You will have to make a second shortcut for the ‘Show’ function as well, as the word changes depending on the current view. You can use the same shortcut for both. Hope this is what you’re looking for. It’s a lot simpler than an AppleScript.
@Trevor: Perhaps that is under Tiger, because I don’t have the “Preview Pane” option in Leopard.