>I tend to be away from UT for long period at a time. It seems like everytime I come back, there has been some improvements and cosmetic changes. :-) This time, I'm curious about the little green diamonds I see in the message headers next to peoples names. What are they there for?

Those, indications from tooltips, will represent if a user is online. Three choices are available, either online, recently online or not online. That can be turned off from your account setup.