i want to know is arcGIS viewer for Flex supported WMS in geoserver?
WMS is supported, and other people have been able to use it with geoserver so it's possible.
Sometimes it can be a little tricky to setup. There can be cross domain hurdles, projection hurdles, specifying the name
of layers to display hurdles, but if you share the URL to your geoserver there are people here who can probably help.