Thanks guys - turned out to be my error. I'd got all the permissions set up, but forgotten that the "registered user" i was using for testing had an overriding access mask on that particular sub-forum, thus making all my testing useless. Works fine now of course.
I'd still be interested in that conditional though, just out of curiosity.
EDIT:
Code:
<if condition="$thread['postuserid'] == $bbuserinfo['userid'] && $thread['firstpostid'] = $bbuserinfo['userid']">
Works if used in threadbit...