It works for me on vB 4.2.0 PL3 but after I saw all the posts on here complaining that it doesn't I went and checked my setup.
It looks like I added this in my postbit_legacy at some point:
PHP Code:
<vb:if condition="$post['lastactivity']">
<dt>{vb:rawphrase last_online_activity}</dt><dd>{vb:raw post.lastactivity_date} @ {vb:raw post.lastactivity_time}</dd>
</vb:if>
Use that and see if it works.