The Flex API forum has long been a place for both the Flex API and Flex Viewer. As of today there is now a separate forum for "ArcGIS Viewer for Flex"
http://forums.arcgis.com/forums/111-ArcGIS-Viewer-for-Flex
If you have questions more specific to the Flex API itself, use http://forums.arcgis.com/forums/18-ArcGIS-API-for-Flex