Influential XML personage James Clark has posted a very carefully thought-out essay on XML namespaces.
Everybody loves to bash XML namespaces, but this essay is the most careful and dispassionate I have seen to date. You should read the whole thing if you deal with XML (or even if you just like to read the product of clear thinking).
Here’s a key quote:
I would claim that the aspect of XML Namespaces that causes pain is the URI/prefix duality: the thing that occurs in the document (the prefix + local name) is not the same as the thing that is semantically significant (the namespace URI + local name). As soon as you accept this duality, I believe you are doomed to a significant extra layer of complexity.