Is it possible to set overlays like polygons and circles behind the roads?
(I think this would be possible only if google sends each tile in two sets one with just background and one with just the roads with full transparency. I am not sure if google does that.
From the api, I could see only ZIndex, which is relative to other overlays. So, it seems it is pretty much not possible. Can somebody please confirm this for me?!
It is not possible - a map tile image contains both the road and terrain and all the other graphic elements of the map itself. There are separate layers for markers and polylines so in theory you could create your own polyline layer containing road polylines and draw under that but that would be too complex. If you would like to draw fully customizable maps check out OpenLayers or http://polymaps.org/ or http://www.cloudmade.com/