If I dive into the forums, going back to older postings and then use navigation on the site or use the browser back button, it works as expected - I return to the list of postings where I opened the tread.
I use Firefox 6.0, this may be browser dependent. I noticed that in the URL
the tstart / start URL parameter seems to store your position (4th page of 30 postings each, for this example).
Hope this helps somehow,