I think there is a bug. When I use toScreenX and toScreenY in my custom MarkerSymbol I always get the marker position on the screen when the map is in default extent. (the center of the map)
I use this to show a InfoWindow on top of my Marker, when I click the marker. (Like google maps, this functionality is missing here in ArcGIS Flex).
When I pan the map, the InfoWindow opens on the "old marker start position on the screen" and not over the new marker position.
If source code is required, I can attach source code.