Nice idea, but it could be confusing for someone who hadn't yet read any of the replies. Ie, they'd see everything in reverse chronological order and be reading the answers before the questions. Most forums I've read stick with a top down, oldest first listing.
What I have done to help this issue is to highlight posts that are new since you last logged in, in green. When you open a green topic (ie, a topic containing new posts, you haven't read yet), you'll see a link at the top reporting something like:
Welcome Drew. There are 3 new posts to this topic since you last logged in.
If you click the "new posts" link, your browser should jump the page down to the bookmarked new entries at the bottom.
Thanks for the feedback. Much appreciated!