That was the problem. Using Image Fade for navigation, I was having trouble using a transparent shadow .png instead of using Sprites for rollover states of menu item. I don't know how, but I found my way to fix it. It was in my mind, and after a lot of changes and thoughts, I found it.
Watch a demo of the effect
I know that the error about showing black when fading out in Internet Explorer a PNG (tested in IE7) was something related to background of parent. So I created a div containing as background the image.