http://www.askthecssguy.com/2008/03/one_pixel_notched_corners_as_u.html
This time with two neseted elements with simple margin and border width declarations. (I had done this before I red the first comment :] )
Not tested in IE6.
.softener-outer {
 border-width:  0.3em  0;
 margin:        0      0.3em;
}
.softener-inner {
 border-width:  0    0.3em;
 margin:        0   -0.3em;
}