Avoid Namespace Prefix:
XML Namespaces permits the
declaration of prefixes for namespaces such as
xhtml; to date
such declarations haven't proven to be very
interoperable in the context of an Atom document.
Here are a few links to prior discussions on the topic:
Employ the use of default namespaces (i.e.,
xmlns=) in all Atom and XHTML elements that appear in your feed.
Not clear? Disagree?
You might be able to find help in one of these fine resources.