If the catalog is WF and doesn't supply a default namespace declaration, then there may be implementation problems. There is no publicly known XML processor that allows you to assert the namespace in this last case. The processor would have to decide at the top what the namespace is. Discussion about whether the processor needs to ask the same question at each element, whether it's in the null namespace or the default namespace. Tony thinks we shouldn't allow people to add prefixes on some things and not others, since he thinks they should be in a consistent namespace.
Norm disagrees as to what is harder from an implementation pov.
Paul is willing to have the default namespace logically be on the catalog element. This implies that this is not the null namespace. An attribute with an explicit namespace prefix is in the global attribute partition. Norm's concern is that we need to cover the ambiguous case of the global attributes. General agreement on this point.