I loved Letterbox, a plug-in that allowed viewing Mail messages in a three-column vertical split window. After I upgraded to Leopard, I immediately tried to re-enable veritical split view. After much failed fussing with strings and MailVerticalSplitView, I decided to try editing the .nib files directly. What follows is a somewhat inelegant hack, but good enough for the time being (or until someone far better at using Interface Builder comes up with a fix):

First, quit Mail, then back up, then go to this directory: /Applications/Mail.app/Contents/Resources/English.lproj. Open MessageViewerContents.nib with Xcode (Interface Builder). Click the divider, and in the Inspector, check the Vertical checkbox, and save. This will enable vertical mode, but cause some issues with the divider still being drawn. To fix this, rename mainSplitterBar.tiff and MainSplitterDimple.tiff to mainSplitterBar-bak.tiff and MainSplitterDimple-bak.tiff.

When you relaunch Mail, vertical view should be on, and it's not too hard to undo. There are, of course, some issues -- i.e. to make the split smaller, you have to change the window size first -- but for the most part, it works great!

[robg adds: While Letterbox hasn't yet been updated for 10.5, there's a new entrant, WideMail, which does work on 10.5. I tested it, and it seemed to work quite well. I also tested the uninstaller, and it restored the standard Mail functionality. Just heed the Read Me's warning about making sure the Subject column is visible prior to uninstalling.]