Namespace name: better to use URN or URL?

["Roger L. Costello" <costello@mitre.org>]

Hi Folks,

Is there any advantage to formulating a namespace name as a URN, vice
formulating it as a URL?

Example:

    urn:publishing:book

versus

    http://www.publishing.com/book

As I see it, 

- the advantage of formulating a namespace name as a URN is that it's
easier to conceptualize a URN as a "name" and not a "location". 

- the advantage of formulating a namespace name as a URL is that the
namespace "could" then have a dual purpose - it could serve both as a
name and as a location to "something".  I am ashamedly ignorant of RDDL,
but I vaguely recall reading something about using a namespace URL to
point to a RDDL document.  Does RDDL enter into this issue?  

My impression is,

A couple years ago there was a general feeling in the XML community that
URNs were the preferred form for namespace names, but recently there
seems to be a shift towards preferring URLs.  Do you agree?
/Roger