This didn’t take as long as I thought it would.
Here is a very simple, non-cute, customizable letterhead template for Mail in OS X Leapord that is suitable for business use. It my first attempt and very simple, but it works. Your logo will be centered in the page. Here is the download link.
Once the file has been downloaded. Put it in the “Resources” folder in whatever category you would like it to appear. Library> Application Support> Apple> Mail> Stationery> Apple> Contents> Resources> Stationery> Contents> Resources for example. To use it, simply drag your logo into the image window and resize.
I’ll work on some more designs later.
Also… you can download: