Originally done as explained in this post, and then updated with better code.
The core of this slanted tab placement is this:
#tabs li.tab a { margin-left: -1px; margin-bottom: -7px; padding: 12px 1.2em 12px 1em; position: relative; bottom: 7px; }
Take a look in the source for the full styling and required hacks for IE.
The same design can be achieved with slightly different CSS code.