OpenLayers: how to style WFS layer with SLD file?

Take a look at OL's "Styled Layer Descriptor (SLD) Example" example. They use SLD to style GML, but should work for WFS (a vector layer) as well.

Credit: OL's Nabble.com forum

Short answer is no - WFS sends you the data and then you are responsible for styling it in the client. OpenLayers uses a stylemap to style a vector layer. See http://docs.openlayers.org/library/feature_styling.html for details of how to style a vector layer.