Win7 64bit Radeon HD 6850
Cycles Render Engine / Image Editor / UV Unwrap : "Correct Aspect" doesn't work if an Image is only been loaded to the Image Editor.
Switch to Cycles Render Engine.
Load an Image with an uneven aspect ratio to the Image Editor.
Add a Cube or Plane to the 3D View and try to unwrap it. Use simple "unwrap" or "project from view", for example.
The unwrapper always unwraps the mesh in a distorted manner, no matter if "correct aspect" is checked or not.
I'm also not sure if any of the other tool parameters is working in this case.
The "Correct Aspect" tool parameter only starts working if the Image in use is also connected to the mesh object in use,
by selecting it as an image texture to the mesh objects material, in the Node Editor, for example.
If you try the same steps in Blender Render, "Correct Aspect" is working with any image loaded to the Image Editor and
with any mesh object in general. No need to set any further connection from image to mesh.