Forum Moderators: open
But in the the forums I participate in (mostly the hands-on coding forums), I often feel that the last post is the topic.
Most technical posts seem to follow a back and forth pattern: An initial question, a follow-up, then back-and-forth between those two posters until a solution is reached; or a similar scenario except several people chime in and a collaborative solution to the problem eventually emerges. In both cases the value of the post is the linear journey the thread takes towards the solution.
That's a fairly long way of getting around to my specific request: show first and last posts when replying, at least in the coding/technical forum categories.
I think the added context would be very useful when crafting follow-ups. I would probably personally answer more posts than I currently do if I didn't have to take the time out to open & switch between multiple tabs to collect the context I feel I need for a quality answer.
And if the posts re-displayed during post preview, that would be even better. I use preview often when formatting code, and it's painful to lose that context. Re-displaying the topic and the _current_ last post would be immensely useful.
In discussion threads, the current "first post only" approach has been a boon for limiting topic drift in the thread. So I use my approach in the technical/coding forums.