Blender 2.64 is now using Color Management tools with sRGB color space enabled by default. It means that Blender will always expect a linear input from YafaRay, which means YafaRay Gamma Output should be equal to 1. This is essentially more correct if we are passing HDR data over the Blender compositor for visualisation & edition. With this workflow, Blender will always show and edit YafaRay renders correctly on the compositor (Nodes and Image Viewer) and will save both OpenEXR and 8-bits YafaRay works with the correct gamma applied each: 1 and 2.2 respectively. Therefore I believe this is the best course of action. It has been already tested for a while and it seems to work.
Since Color Management with sRGB will be default in Blender from now on, future releases of the YafaRay exporter should have default Gamma Output changed to 1, in order to spare unnecessary confusion to our users.
Pd: YafaRay integration in the whole Blender linear workflow might need integral revision to be sure the pipeline is well tuned for linear work in both YafaRay and Blender.