From FreeBSD 5:

From FreeBSD 5:
  Bring eventhandler callbacks for pf.
  This enables pf to track dynamic address changes on interfaces (dailup) with
  the "on (<ifname>)"-syntax. This also brings hooks in anticipation of
  tracking cloned interfaces, which will be in future versions of pf.

The events are called attach and detach instead of arrival / departue.