Are you using IE or firefox for a browser?
If you use firefox and have the greasemonkey plug-in, there is a script for RHP that does what you are asking about, plus a little more.
If you look in the general forum, probably on page two or three by now, there is a thread that talks about dots. Go through that and you will see what I'm referrring to.
Basically what it does is changes the color of the dot in front of the thread to indicate whether or not you have posted in it. Also the thread is in bold text if you have not opened it to read it. It will change to normal text once you have opened it, and then change to bold again if there is a new post since you last looked at it.