For the last week my battery indicator and bluetooth icons had been missing from my top panel in Ubuntu. When I did Bluetooth stuff the pop-up "speaker balloons" from the bluetooth applet would still appear at the corner of my screen, but the Bluetooth applet itself was "invisible". I finally figured out that what I was missing was a "Notification Area" for these applets to "live". Right-Clicking on the Panel, Selecting "Add to Panel" and then adding the "Notifcation Area" (from the Utilities category) fixed my problems. Indeed, I then found out that the Software Update notification icon had been trying to tell me about software updates, but I hadn't seen it, so I had to install several software updates!