In the guide, I believe this is no longer true for 3.1:
"This would look for a partial named _link_area.html.erb and render it using the layout _graybar.html.erb. Note that layouts for partials follow the same leading-underscore naming as regular partials, and are placed in the same folder with the partial that they belong to (not in the master layouts folder)."
In my testing I've found that Rails 3.1 looks in the master layouts folder with underscore, eg "_bar.html.haml"