This tutorial explains the workflow and how to setup a tree and so that it bends in the wind in Unity by using the ats mobile foliage shaders and Blender for the vertex painting part. This works by giving the mesh different vertex colours. The result will bend then by the ats wind script or the T4M Terrain system wind settings, dependand of the painted colour.
Unfortunately you need a older version of blender for this task too. Gradient vertex painting exists in Blender 2.49, but is still not converted to 2.6x series. I hope that this for this workflow important script will make its way into Blender in the future. For now we need Blender 2.49 though for the gradient part.