I think that the default style of pre elements in Vector looks a bit deprecated. I noted that after seeing how Commons has customized the display of this, which I believe is worth consideration for the MediaWiki core IMO, too. In the Monobook skin, I think the dashed border is quite ok, but I believe that Commons' way of doing this fits better into the Vector skin than Monobook's implementation of this.
The original idea comes from Diebuche, http://commons.wikimedia.org/w/index.php?title=MediaWiki:Vector.css&diff=44847269&oldid=43480117
attachment css.patch ignored as obsolete