Just a very quick message for now to say that I have been working on implementing a sorely needed feature in v3 of the GMap API. In v2, both GPolylines and GPolygons allowed the user to add to/modify the shape by calling the enableDrawing and enableEditing functions. This hasn't made it yet to v3, so I have gone and written my own. A demo can be found at: http://www.shanetomlinson.com/static/gmapV3EditablePolys/html/test.html.
It still needs a lot of work and right now only works with a Polyline, but I'll get there.
Right now it supports enableDrawing, enableEditing, disableEditing, you can refer to the GMap v2 API documentation to see the arguments these functions take.
The source has been uploaded to GitHub at http://github.com/stomlinson/Polyline-enableEditing-enableDrawing-for-GMaps-API-v3