I've got northing and easting in lat long for 4 points and would like to convert these coords to web mercator auxilary sphere and show on map as a polygon using flex api. Do I need to create and publish a geometry service to do a projection and then read the geometry back? -- This will need to be done repeatly in an automated fashionand I was hoping to not have to create new gis layer each time the calculation was performed
Well, you could publish a simple geoprocessing service that uses the Minimum Bounding Geometry tool with either CONVEX_HULL or RECTANGLE_BY_AREA option. Set the Output Coordinate System geoprocessing environment variable to web mercator. This will force the output of the Minimum Bounding Geometry tool to be in web mercator. Your service would return the feature class output by Minimum Bounding Geometry tool.
As far as projecting on the client side, I can't help you. I'm thinking you'll need a service.