I'm using Chrome 17.0.963.56 m (latest update) and notice a strange border artifact for dialog headers.
You can also test this out by navigating to the dialog docs and clicking the "Custom Overlay Swatch" example.
This may also be a duplicate of #3187.
It looks like it goes away if you add a border: 0 to the .ui-dialog .ui-header style. Working on a pull request now.
Closing because of new issue ( #3627 ) created from pull request with possible fix.