encoding itunes dtd..

Quote it.

xml.rss "version" => "2.0", "xmlns:itunes" =>
"http://www.itunes.com/dtds/podcast-1.0.dtd"

Or even,

xml.rss "version" => "2.0", :"xmlns:itunes" =>
"http://www.itunes.com/dtds/podcast-1.0.dtd"

In most cases in Rails, a string and a symbol is interchangeable. (The
second version preserves it as a symbol)
Vish