Besides the "base URI", your namespace URI has structure:

• http://www.w3.org/1999/xhtml

  Means: "xhtml published in 1999 by the W3C"

• urn:publicid:IDN+cde.berkeley.edu:schema:coursedoc:course:200401:en

  Means: "The English version of the course schema published in 01-2004 by the CDE at Berkeley "

I recommend using a URN rather than a URL (e.g. http://somewhere.com/).