Are you talking about the slim blue navigation bar? If so, poking around the source I found this:
-moz-box-shadow: 0 3px rgba(0, 0, 0, 0.1), 0 -2px rgba(0, 0, 0, 0.1) inset;
This is what creates that subtle drop shadow effect.
The source is very interesting though, I'll be exploring for awhile.
EDIT: I created a quick JSFiddle for it here: http://jsfiddle.net/qGDes/1/.