Home page: http://live.gnome.org/Istanbul
Istanbul appears to have been abandoned. It is no longer available in the Ubuntu repositories and its official web page is no longer accessible.
Based on gstreamer, written mostly in python, Istanbul hasn't been updated since 2007. Once started, it sits in the system tray as button waiting to be pressed in order to start recording. To stop recording simply tap the button again. Istanbul creates ogg/vorbis encoded videos which can be very quickly played back and/or distributed.
$ sudo apt-get install istanbul
Applications --> Sound & Video --> Istanbul Desktop Session Recorder
Tips: Set your screen resolution low to ensure a reasonable recording frame rate.