The plasma-widget-message-indicator is not new to Kubuntu, but it got some nice improvements in this cycle. It got a visual overhaul to fit the new visual theme of KDE 4.5 (and in my opinion looks generally more pleasing to the eye), but the new killer feature is one you can’t see at all.
Normally, when you click on the indicator widget in the systray, it opens a menu and you can see the backlog of information for each application that uses it. If you shift-click instead of just click, you don’t get the menu. It takes you straight to the application where the most recent event happened. This way if you see a notification because you got highlighted on IRC (for example), you can shift-click and go straight to it without having to stop at an intermediate step. Based on this new capability, we decided not to show an application systray icon for Quassel (our default IRC client in Kubuntu).
P.S. The screen shot is from the KDE Plasma Netbook workspace, so the panel is on top (the image is not upside down).