Lapizuli - 5:56 am on Aug 27, 2011 (gmt 0)
As a user, I've noticed this happening recently - I land in the middle of some discussion thread. It's a bit disconcerting but I think I like it. When I'm sent to the first page, I rarely bother clicking through to try to find my relevant terms.
I guess it also depends how relevant the topic is. If I'm looking for ways to fix blue widgets and come to the first page of a thread on fixing blue widgets, I'm good and will read page after page. If the thread is actually about red widgets and someone mentions blue widgets only on page 6, I'll never see it - I'm out of there if I've landed on page 1 because I feel like the search engine's job is to get me to the specific page I need, not the general site. On the other hand, if I land on exactly the page with the blue widget information, after I read it I look around to try to find a reason to stay, because hey - these might be my people.
One thing you might be able to do is provide better navigation aids, so your visitor can see where they are - that it's a forum thread, that there's a beginning that's easy to get to - some way to ground them. Low clutter, especially, so they won't feel like they've stumbled into the middle of a hectic party of strangers and bow out with an embarrassed "excuse me" (which may be what the bounce is about).