Jon Udell writes about RSS, RDF, and XML in general...Stefano's formulation suggests to me that the troubled relationship between RSS and RDF may have been a red herring all along. Either we do or don't need some higher-order model to manage mixed namespaces sanely. Nobody knows yet. That the question arose in the context of RSS may simply have been an unfortunate historical accident -- RSS happened to be a likely candidate for the necessary large-scale experimentation, and got caught in the crossfire...
Shouldn't we then substitute XML for RSS 2.0 in that sentence, and say there is no consistent way to interpret material from other namespaces in any XML document, period?
Shouldn't we then say, there is no reason to create any mixed-namespace XML document that is not RDF?
This is beyond RSS, beyond RDF, even beyond XML.
This is known as The Symbol Grounding Problem.
Namespaces allow you to create unique (enough) symbols. There is no consistent way to interpret them. All XML-based standards should fully support namespaces. The minimum acceptable standards should support distinguished symbols from among various standards.
What those mixtures "mean" in any specific context will be, well, context dependent. All we can hope for from XML alone is an arrangement of symbols. Whoever tells us of the arrangement will also have to tell us how to interpret the arrangement.