CSS box-shadow hidden (z-index does not fix)
You need to define positioning for #primaryNav. Z-index only affects positioned elements.
I just added this in firebug and it fixed:
#primaryNav {
position: relative;
}
I would avoid using a negative z-index. Simply change z-index of #page to 0.
As jlego already said a relative position should fix it.
By the way I would suggest ensuring that there is no shadow left or right of the #primaryNav. Since #primaryNav has a width of 100% a shadow on the side makes a horizontal scrollbar appear.
For fixing this you could set a overflow:hidden to #iframe