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"