Seems like there's a bug in the way Webkit is rendering partially transparent borders. Where the corners meet it looks like the borders are overlapping. see: http://timefortats.com/webkit-border.html (or this screen shot of said page: http://www.flickr.com/photos/whole/3237146877/)
FF works as I'd expect, though I wasn't able to track down expected behavior in the spec.
*** This bug has been marked as a duplicate of 21835 ***