In /actionpack/lib/action_controller/vendor/html-scanner/html/sanitizer.rb there's this:

Specifies the default Set of tags that the

sanitize helper will allow unscathed.

self.allowed_tags = Set.new(%w(strong em b i p code pre tt samp kbd var sub sup dfn cite big small address hr br div span h1 h2 h3 h4 h5 h6 ul ol li dt dd abbr acronym a img blockquote del ins))

I can't think of any reason to allow dt and dd tags and not dl (in fact, stripping out the dl tags leaves you with invalid html), so I'm wondering if that's just an oversight.