White House Blog Feed
Valid, with warnings. Notably:
Content-Type: text/xml - technically illegal. Content contains a “curly” apostrophe properly encoded as utf-8. Does not tend to cause a problem in practice.
Two entries with the same id - technically legal, but in practice an egregious error.
In related news, http://www.whitehouse.gov/ is well-formed XHTML 1.0 Transitional, served as... you guessed it... text/html. This time with charset=utf-8. Again, nothing that causes any problems in practice.