Dumb Question

If I have a default xmlns declaration xmlns="http://xhtml" as opposed to
xmlns:xhtml="http://xhtml", how do we emulate default prefix in RDFa?

Meaning if I have a property <span rel="foo"> does it resolve to the
default prefix or to the current page? Is there such a thing as <span
rel=":foo">

Am I missing something?

-Elias