Blender's and Xnormal's standard MikkTSpace tangent space normal maps are not rendered correctly, likely a problem with the tangent space calculation.
Images attached, BI vs Yafa comparison.
Wanted to attach .blend, bug tracker won't let me - "The selected file yafanormaltest_s.zip could not be uploaded. The file is 667.29 KB which would exceed your disk quota of 1 MB."
Windows 7 x64, Yafaray 1.5.0 Stable, Blender 2.67b official