Just like dump_node*, this method requres the node as an argument.
Like this:
$removeThis = $dom->get_element_by_id ('26');
$unlinked = $removeThis->unlink_node($removeThis);
In the example above, $unlinked seems to be NULL.
I'm using 4.3.0-dev (build: Nov 14 2002 10:13:12) in Module mode on Apache/1.3.24 - Win2000.