I put this question in the moodle.org support forum and received a reply suggesting I send it to this Tracker area.
"I know that it is possible to create a static introduction to a forum, but what I'm looking for is something similar for each thread. The reason is that I create summaries of discussions in each thread. The aim is to save people having to scroll through long discussions if they want a quick way of seeing how a matter was resolved. (I've often thought this would be really helpful in these Moodle discussion forums to have a summary at the top of the thread - of course it requires someone to write it!). At the moment I create summaries as wordprocessed documents and upload them next to the forum. This looks a bit clumsy and is time-consuming both for me and for the users.
A friend said this is described in other forums as being a 'sticky' or something like that, used by moderators to give some extra info.
I'm using Moodle 1.6 but due to upgrade to 1.8 in case that's relevant and the site page is that http://www.communityradiotoolkit.net/communities/course/view.php?id=2
Many thanks for any advice coming this way."