The Visio 2007 Viewer Developer Reference is up on MSDN and can be accessed at: http://msdn.microsoft.com/en-us/library/cc297217.aspx.

The reference includes the full API reference and some short samples on how to embedded the viewer in different environments.  The Viewer provides an application programming interface (API) that enables solution developers to perform the following tasks:

• Load and unload Visio drawings
• Select shapes
• Follow hyperlinks
• Display Viewer dialog boxes to the user
• Customize the size and position of the Viewer window
• Customize the user interface by changing foreground and background colors and displaying or hiding the grid and the scrollbars
• Control the color and transparency of layers in the drawing
• Control the color and visibility of reviewer markups (comments)
• Customize the toolbar by adding or removing buttons
• Respond to user actions in the Viewer interface