--- Operating System, Graphics card ---
Windows 7 (on an iMac), Mac OS X (Macbook Air)

--- Blender version with error, and version that worked ---
2.67a with error. Don't remember when this last worked as expected, but I'm sure it did at some point.

--- Short description of error ---
When a mirrored mesh is parented to an Armature using automatic bone weights, deform weights are set correctly. However, when viewed in weight paint mode, display shows weight paint values also mirrored, incorrectly representing the bones' actual influence on the mirrored mesh.

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

See attached image. Create mesh, add mirror modifier, add armature, parent mesh to armature with automatic weights.