Hmm - even post 4.0.15a I've had one random thread deletion :\
So, I wrote a little thread recovery script anyway in case I have to do this when I don't have much time.
Takes a threadid and forumid arguments and then recreates the thread record from existing post records. Everything apart from the number of views can be accurately re-created (I set number of views to the replycount * 10 as a rough value).
This script hasn't been tested extensively so it probably doesn't handle a bunch of exceptional conditions. It does do what I need it to do though
btw, you have to stick it (threadrecover.php, attached) in your .../forum/admin directory.
Michael.