Fracturing of identifiers (How many "The Matrix" are there?)

dmoz.org has identifiers for movies, and so does tap.stanford.edu.  For example,
dmoz.org uses "Top/Arts/Movies/Titles/M/Matrix_Series/Matrix,_The"
and tap.stanford.edu uses "http://tap.stanford.edu/data/MovieMatrix"
and there may be others.

I don't see any of these sites using triples that say one is owl:sameAs the
other.

Who is actually supposed to be responsible for sameAs assertions?  dmoz.org?
TAP?  Or some third party?
Is this in practice a showstopper for useful semantic web searches?
(This may be FAQ but I couldn't find it.)

Thanks,
- Jeff