Solved! Go to Solution.
ArcGIS 10.1 can create dynamicly rendered maps on the server.
http://resources.arcgis.com/en/help/flex-api/concepts/index.html#/Dynamic_Rendering/017p0000002q0000...
Before 10.1 you had two main options:
1) Pre-create the thematic maps ... with the limitation that you couldn't change the rendering.
Examples: http://services.arcgisonline.com/ArcGIS/rest/services/Demographics
2) Do the rendering on the client. Using feature layer (not feature service).
Examples:
http://resources.arcgis.com/en/help/flex-api/samples/index.html#/Based_on_MapServer_layer/01nq000000...
http://resources.arcgis.com/en/help/flex-api/samples/index.html#/Using_Renderers/01nq00000052000000/