Indeed, the icons reflect any changes / new messages since your last logon and if you look carefully at the legend at the bottom of the page that's what they are supposed to do. I agree with you guys that it could come in handy to only reflect messages you actually did not read. Although for people who never read all the messages (probably a lot) it would be a real pain if this was how the board worked, so there would be a need for users to set their preferences on this in their profile.
I'm planning on upgrading the board software (phpBB) this year and for that I first need to upgrade our hosting account. The database will get a bit larger with the new version and I'm already operating on hd-space bounds for this account. Upgrading also means money so I'll need to get in touch with our sponsor and this all might take some time.
When I setup the new board software I'll also see what mod's are available that might implement the requested features.
One thing is for sure, if there isn't a mod available, I won't be the one who's going to program it. ;) There's life besides Internet and the board you know.
