Great! Prior to Leopard Mail I'd been using Aaron Harnly's Letterbox, it did the basic layout well but broke after doing a search, although that was easily remedied, but as of writing, Letterbox hasn't been updated yet.

Installation of Phatso is easy enough, perhaps a little bit daunting for someone new to Mac or even Terminal.

So far it looks good, it remembers the layout between restarts and searching doesn't break anything. Many thanks to Vernon Tang, the author of this plugin!

Great to see an attempt at this for Mail 3, unfortunately this stopped Mail from starting for me.