In 2012, the Revit Viewer 2012 is the Demo/Viewer option from previous releases, same functionality, just a different way to access it (you will still get into "Demo/viewer" mode if you are using network licensing but cannot pull a license). This will have its own icon on the desktop.
The all functions of Revit are available with viewer except below:
- save or save as in all cases
- exporting or publishing modified projects
- exporting or publishing any projects to a format containing model data that can be modified
- printing projects after changes are made