Just in IE6 & 7 (of course). See on this page:
http://www.mattpealing.co.uk/concept/dropdown/

I have some dropdowns in the navigation that are supposed to overlap the jQuery banner beneath.

I've set the z-index of '#nav ul li .mega-menu' to 10 so that it should overlap the jQuery banner (some of the elements within that are given z-index values of 5 by the jQuery script).

So I cannot understand why this isn't working.

Does anybody have any ideas why IE doesn't understand how I've set up the z-indexes? I know it does struggle sometimes, but it's not an amazingly complex layout.

Fixed it by increasing the z-index of #nav