--- Operating System, Graphics card ---
Gentoo Linux x86_64
Kernel version 3.9.8
NVIDIA GeForce GTX 560

--- Blender version with error, and version that worked ---
blender.org build version 2.68a

--- Short description of error ---
When there are no faces in a mesh (as when working with the skin modifier) and the mesh has a mirror modifier, vertex normals on the mirrored copies of vertices point in the same direction as their source verticies, rather than being mirrored.

--- Steps for others to reproduce the error (preferably based on attached .blend file) ---

From a factory default file:
1. Enter edit mode for the default cube.
2. Turn on display vertex normals under Mesh Display.
3. Select all vertices delete them.
4. Add a mirror modifier.
5.Turn on "Apply modifier to editing cage..."
6. Create a single vertex.

The normals for the new vertex and its mirrored copy will be pointing in the same direction, rather than opposite. This will continue to affect other vertices until a face is created. Recalculating normals does not correct this, but applying the modifier will result in correctly mirrored normals.