The US government have published a draft of an interesting document: Recommended XML Namespace which will be a policy document for defining namespace URIs. It's a good, in-depth review of the various possibilities such as using a single namespace vs. multiple namespaces. They've decied to opt for URNs over URLs for their namespace identifiers which is probably a good move as a long term strategy. For us quick-n-dirty types, HTTP URLs are the only practical choice given current Internet infrastructure constraints.