Win8 64 bit, 560Ti
Short description of error
Unwrapping doesn't take into account mirror modifier geometry. What this means is that unwrapping only takes into account the "real" geometry, so for example if you unwrap a sphere with a mirror modifier you will unwrap one half of the sphere which gives severe UV distortion at the mirror axis. This of course kind of makes sense because the geometry isn't real (and you can only select half of the mesh), but it would be nice if Blender could act as if the mirrored geometry was there so you could get a better result. After all with the mirror modifier you're probably looking for an unwrap that also has mirrored UVs, so the less distortion the better.
Here's an example, mirrored to the left:
Exact steps for others to reproduce the error
Create two UV spheres.
Delete half of one UV sphere and add a mirror modifier.
Add a seam parallel to the X axis on both spheres.
Unwrap. They will look different which is unwanted behavior.