4.5 makes Fantom occluding on hair much easier
Back on the old forums I Had a topic about using Fantom shielding which is a a mesh with the Ubersurface shader on it to cause occlusion shading on transparency heavy objects(such as hair) and there surroundings.
The Geometry shell option in 4.5 allows you to create a duplicate of the original hair mesh slightly offset which you can use providing superior results.
Basically this lets you render hair with occlusion effects significantly faster (6x +) and with the geometry shell its now faster to set up.
Heres a quick render