I've already had a feature request for this on a forum I just installed. I'm not sure why, but I guess somebody cares about it. How hard would it be to query a session log or such to see which users are browsing the current topic or forum?
Enable Who's Online
You can enable the function to see how much users are currently browsing the topic and you can show their user names too. That's exactly what you are looking for. Must be in the board settings I guess.
At the moment there are two places where you can see the users that are reading a topic: the page http://www.elkarte.net/community/index.php?action=who and enabling the "Show who is viewing the board index and posts" in the theme options page.
Do you mean something else?
P.S.
It's not necessary to prefix the topics with "feature request", it's the feature discussion board, so it's fine. ;)