I was editing my Rails application layout file in Vim 7.3 with this vim-slim plugin installed and noticed that Vim was using a filetype of HTML instead of Slim for my application.html.slim file and not providing syntax highlighting.
My other views that followed the *.html.slim naming convention were correctly using the Slim filetype and syntax.
The culprit seemed to be the first line of the file, which was doctype html. I changed it to the equivalent doctype 5 and now Vim correctly sets the filetype to Slim for this file.