How very dumb of me… yes, it’s my custom theme. I could have sworn when I switched to Twenty Eleven with the same code I had the same problem.
Thanks for taking the time to reply.
That’s just it: the meta box isn’t added. I filtered the arguments used by bbPress in register_post_type and dumped them to a file – I can see that thumbnail is there in the supports array but the meta box doesn’t appear.
I notice that I can link media items to forums, and this is shown in the media library as you would expect for posts. However, has_post_thumbnail() still returns false for the forum.
I blogged about this little tweak too and someone has just posted an edit to display a friendly message instead of the standard “not found” which sort of completes it 
http://philipjohn.co.uk/2011/11/14/hiding-bbpress-topics-from-logged-out-users/#comment-1885
I’m not changing the message at all – hence the “dumb” behaviour.
