Take a look at the following website: http://driz.co.uk/mars/index.html
If you look at the jQuery slider thingy at the bottom right in IE6/IE7 you will notice that the orange arrows appear behind the white gradient overlays even though the z-index of the arrows is set to higher values than that of the overlays.
It works fine in all the other browsers I've tested it on, including IE8 just 7 and 6 display the indexing wrong. Any ideas? Need to get this project out asap. Thanks.