Facebook Canvas-friendly Theme
My thoughts in a nutshell:


Start with the Zen base theme
Sub theme this with your custom theme (there are instruction in the Zen theme on how to do this)
Use a fixed width option, and resize the main column to 520px
Add CSS to spice it up and give it colours etc (out of the box it is fairly boring colour wise)


In order to hide breadcrumbs you can override the page.tpl.php to not display them.