As I understand it, the light bulb icons are suposed to show yellow for a forum only when there are new, i.e., unread, posts. I find that the light bulb icons on my board show yellow when I log back on, even though I've read all the messages in the forum. Is this a glitch of some kind?

The new post indicators aren't that smart - they turn on when the forum has new content since your last visit to the board started, but they won't turn off when you read something.

My problem is that I can visit a forum, read all of the messages for each topic, log off, then log on again later and find the light bulb still lit for that forum, even though there's no new content. What does make the light bulb turn off?

Visit My Profile, and click on the Update Post Indicators link, and that should make all of the lightbulbs turn off.