I’ve never been thrilled with Mac Mail’s default layout.
Letterbox is a free Mail plug-in that gives you a three-column view that’s more logical for wide screens. It rearranges the user interface into three vertical columns, so the message pane is to the right of the message list, instead of below it.
I’m running the latest Beta for Snow Leopard, and it works great. +1, highly recommended.
Seek Nuance 
I think the divider has to be horizontal, so the message appears at the bottom.. it is most common view.. and I really used to it.
I disagree, Mac How. If the message list and the current message are stacked, then they’re sharing the height of your monitor between them. The huge width of a widescreen monitor is wasted on them – messages are unreadable with such a wide wrap.
In 3-column view, you have a nice long list of messages, and the current message is narrow enough to read comfortably and high enough that you don’t need to scroll as much.
Give it a go if you have a big monitor – it really is a big improvement!
I liked Letterbox on 10.6.6 BUT……..
It doesn’t work on the latest Snow Leopard 10.6.7 released today..
My Mac has automatically unloaded it…………… says it’s incompatible …….pity……….
Amazing what a Google search will uncover. 🙂