Huge thanks go to Sam Spilsbury for generally being awesome but more specifically for adding the invisible-window-borders and state-dependant-shadows to unity-window-decorator and also the ability to tweak these through the theme.
You can see that Radiance needs a tiny amount of work to remove the border around the title-bar that makes it look like the there is a size mismatch, I'm hoping that will get fixed soon.
N.B. Remember this is purely visual, there is still a nice big hit areas on the left, right and bottom edges for you to resize the window with, you just can't see them :)
N.N.B. We'll try and fix http://launchpad.net/bugs/710356 before beta.