It appears that #8174 on edge broke the ability to render HTML in the JS respond_to block for an ajax request. Line 131 of mime_responds.rb explicitly sets the template_format to that of the mime type. This forces action view to only look for templates of that format (including layouts and partials).
In my case, I have a link_to_remote that I want to return HTML for: http://pastie.caboo.se/76204
I could see other situations where I don't necessarily want to force the template format based on the mime type, such as wanting to render HTML partials inside of an Atom feed.
In my option, it should use the order of the Accepts header to give preference to template formats just like it does with choosing the format for the respond_to block. What are your thoughts?