This is very tricky stuff. There is a field in the posts table called post_position… but bbPress basically ignores it at this point, and uses the post date instead to sort the posts in order.
But one day, splitting or merging topics may be supported in the API and post position will become important again. Then any posts you split or merged to another topic (even using a plugin) may no longer be in the right order!!
Note: since there’s no clear API support yet and I don’t want to mess up my posts database, what I do instead is grab the text and repost it manually as a new thread. Then I use ck’s “edit post attributes” to change the author from my username to the original author.
This is, and has been, my Number One issue with BBpress since 0.9.
The inability to actually administer my own forums is sure;y higher priority than Email Notifications, Facebook, Smileys, YouTube integration etc. Reading Matt’s IRC transcript and Trac, apprently not.