I have a flex viewer 3.0 map in wgs 84 (4326) (ESRI basemap) with a wms which is 4326 as well. for some reason the wms doesn't overlay correctly. it is way off. appears on the maps but it is flattened quite a bit.
I would try again with Spherical Mercator, I haven't used the ESRI base maps but in web mapping EPSG 3785 is the de-facto standard and likely the source of the problem.