When you created 500 as Archive, did you create 501 as Archive_talk? I'm just wondering about this after reading http://www.mediawiki.org/wiki/Manual:Using_custom_namespaces because it sounds as though I should have been asked if I wanted to move the associated talk pages, and I wasn't asked. Of course that may be because it ignores empty talk pages.
I came across this on the Mediawiki list, and though that it might be related? <quote> Don't write anything like: $wgExtraNamespaces[NS_FOO_TALK] = "Foo talk"; # no underscore
but: $wgExtraNamespaces[NS_FOO_TALK] = "Foo_talk"; # with underscore
See http://www.mediawiki.org/wiki/Manual:Using_custom_namespaces </quote> Could be a total "red herring" of course