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-top: -7px;
padding: 12px 1.2em 12px 1em;
}
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.