Originally done as explained in this post, and then updated with better code.
The core of this slanted tab placement is this:
#tabs li {
float: left;
}
#tabs li.tab {
float: right;
}
#tabs li.tab a {
background: url( images/toptabr.gif ) no-repeat 0 0;
margin-right: -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.
There is also a left-2-right variant.