http://moodle.org/bugs/bug.php?op=show&bugid=122
Currently, the feature just moves the whole topic instantly, and no "trace" is left behind in the original location (except for an entry in the logs).
A possibility is to leave behind some sort of unreplyable post that indicates the topic has been moved, and provides a forwarding link, but I'm undecided if this is necessary and just clutters the forums up.
Another future possibility is to be able to "break" off a sub-thread and promote it as a whole new topic.
Responses on these are welcome.