An XML namespace is not a URL, its a URI. If it looks like a URL (http://mycompany.com/food/thought), it should resolve to an RDDL instead of 404 not found. If it is not a URL it should be a URN (urn:mycompany.com:food/thought).
O'Reily on RDDL has a good document to get you into the headspace.
www.rddl.org has the RDDL specification.
Since a URL is tied to a companies web deployment, i'd structure the URIs such that they dont collide with web content. The webserver should map the final part of the namespace to an RDDL file.
namespace URL: http://frogs.com/uri/food/flying-things/1.0
One thing... security. I dont want to let the rest of the world know my internal protocols and schemas. This could be enforced by a secure site (internal.site.com) or protected areas on the site.