I think that the forums interface has some core flaws when it comes to big/active threads.
A couple of common headaches:
A) There is no way to quickly scroll to the newest messages. If I am reading a thread that has hundreds of messages, I have to scroll up and down to find the new message(s). This could be handled much more nicely by putting in a button on the sidebar that automatically scrolls to next/previous new message.
B) If I open a busy thread and start reading, it might take me 10 or 15 minutes to finish up. And maybe during that time I'll feel like posting a reply or two. Now when I go back to my profile I see that I'm the last poster, but I've missed any messages posted while I was reading the thread. And if I go back in to look for interim posts, ALL of the messages open up, making it impossible for me to track activity.
This second one is not a difficult problem to solve -- change the internal timestamp you keep from the time the user posted to the last time the user accessed the thread. That should fix the problem, at the expense of showing the user his or her own messages as "new." If you want to fix that, some additional logic could be put in place where the timestamp would be updated to the posting time only if no new posts from other users were done in the meantime. Again, not at all difficult, assuming that you have a reasonable data model backing this thing.