Screenshots have been floating around for a while with mockups of a three-paned Mail.app à la Outlook.
But now there is a hack that actually works.
A member of the Ars Technica Open Forum, Mithras, has posted a hacked binary of Mail.app, which displays the preview screen on the right, giving Mail a “widescreen” form factor.
And it actually works:
Click image for a full-sized view
Mithras’ binary has a different name (MailWidescreen) so you can copy it into your Applications folder without fear of overwriting your default copy (although backing up is always, always wise. UPDATE: Always!). Then just open the one you want each time.
Unfortunately, you cannot yet manually resize the middle column, although Mithras promises a fix for this within the week.
There is however a work-around. The middle pane of the hack takes its size from the depth of the Message List in your normal copy of Mail.
So, if the middle column is too small, open your default copy of Mail.app and increase the depth of the Message List. Then the middle screen in MailWidescreen will be correspondingly wider. Make it wider than you want. Then it will reduce to the width you actually want as you resize the whole window from the bottom right.
You can read the whole thread 
on the Ars Technica Openforum or cut to the chase and… download the binary.
UPDATE: Mithras works fast. He has now packaged it in the form of a classic Mail.app bundle , which you can drop into your ~/Library/Mail/Bundles folder where MailTags, Mail Act-on, Mail.appetizer and the rest live. And just as easily take it out again. Extra bonus: the middle column can now be manually re-sized as normal.
[Thanks, Nick]
There’s hope for Letterbox:
http://harnly.net/2007/software/letterbox/letterbox-for-leopard-its-coming/
[...] a whole bunch of other folks (43Folders, Hawk Wings – 1, 2,), many of them far more Apple-savvy than me, I have downloaded and installed the Letterbox [...]