Building upon Trevor's initial design, here's my take, which looks less bloated and offers a more consistent look. The .diffchange spans no longer have a left/right padding or border, ensuring proper display in Chrome, while maintaining proper word spacing.
(The dark 2px wide spaces are an unintended side effect of Chrome's border-radius rendering. Firefox shows only top/bottom border.)
http://windekind.demon.nl/images/Diffs-1.png
Thoughts?