When it comes to XHTML 1.1, the document’s content type should be application/xhtml+xml. This plugin can be used change the content-type that WordPress serves the document, and also change the Character-set and the XML declaration.
“This is a plugin for WordPress which allows you to serve your WordPress content in different content types with fall-back option for backward compatibility.”
See more info on mymime
Eep, a competitive plugin to mine!