My humble visibility analysis python plugin for Quantum GIS can be downloaded from the User Contributed Repository. It is currently experimental software (version 0.0.9), but new features will be added soon according to my thesis work.
You can find it under the name of "Visibility Analysis" as seen below:
It will be installed under the menu directory 'Visibility Analysis' and it can also be invoked by pressing F7 key or by clicking the 'binoculars' icon as a shortcut.
The plugin can also be downloaded from the url: http://spatialserver.net/pyqgis_1.0/contributed/vap.zip
I'll try to give the details including the development efforts for the plugin in a future post. For now, take care...