It appears to be impossible to alias the main namespace. Try this:

$wgNamespaceAliases = array(
'Foo' => NS_MAIN,
'Foo_talk' => NS_TALK,
);

The alias "Foo_talk" works, but "Foo" is not recognized as a namespace in links, URLs, or search.

Version: 1.15.x
Severity: normal
OS: Linux