Here we go 
I cut my default cube, dissolved edges (including vertices), marked seams, unwrapped.
As you can see on the following images, the texture on the mesh appears to be fine, however UV wire shows some weird artifacts:
Why? How to get a rig of those? Thanks.
Noticed the same thing when unwrapping a model with ngons.
Strangely the texture as you noticed on yours looked fine on the model.
Very likely the same thing as this bug report :
http://projects.blender.org/tracker/index.php?func=detail&aid=31074&group_id=9&atid=498
I hope they will do a bugfix focus instead of new features before 2.64, as they’re pilling up on the tracker since 2.63 release.