I'm working with the ArcGIS Flex API. For example, ArcGISDynamicMapLayers do reproject their data in order to make it in the same projection as the map. Do FeatureLayers have the same behaviour?
The answer is Yes.
Have a look at this sample: Generalized Data.
If you have a look at the Original Services, the Feature service is wkid:4267, while the map and the tiled map service are in wkid:3857/102100.
If you have a look at the service call using Firebug, you will see that the data is requested in wkid 102100: