In some cases I know they've logged in since I replied, because I replied to later messages from them as well. I strongly suspect a lot of people don't know how the system works or don't know they can get a reply through it. The purpose of this thread was to raise the level of awareness of the system and the fact the yellow around the Mail link means they have a message to be read.
Hmmm, it seems to me there is another explanation for this possible:
At least with my browser (Opera, don't know for others), I can log-in, get to multible stories – all in separate tabs – start to read one and send you a mail concerning this story. As long as I don't log-out, close and restart Opera, or restart Windows, the tabs I created for your stories are not updated! So I can't see I got mail.
My system is quite stable and I run it 24/7. So if I come back to read your story on the next tab, I may send you another mail concerning the second story without realising there is an answer from you for my first mail.
I've not tested this but I know the reverse happend to me.
After a restart of Opera every tab showed I had mail. I read the mail and then changed to another tab which still showed I had mail. Instead of reloading every tab I restarted Opera again so it updated all SOL tabs.