I'm using a brand new installation of Sky with no custom css. This is the css produced when I set the main content area's corners to the default setting of 6px:

.rc-6 #columns {
-moz-border-radius: 0 6px 6px 6px;
-webkit-border-top-left-radius: 0;
-webkit-border-top-right-radius: 6px;
-webkit-border-bottom-right-radius: 6px;
-webkit-border-bottom-left-radius: 6px;
-khtml-border-radius-topleft: 0;
-khtml-border-radius-topright: 6px;
-khtml-border-radius-bottomright: 6px;
-khtml-border-radius-bottomleft: 6px;
border-radius: 0 6px 6px 6px;
}

This looks pretty much like what you'd expect it to look like, no rounding in the top left corner.