[jQuery] animate() not working for opacity in IE

[Brian]

I'm trying to animate a div from 100% opacity to 40% opacity WITHOUT
using fadeTo(). I need to use animate().

It works fine in chrome/FF/safari, but in IE, the opacity doesn't
animate, it simply changes to that after the animation is done.
Happens in IE 7 and 8. I'm doing this:

                    .animate({
                        width: new_width,
                        top: new_top,
                        left: new_left,
                        padding: new_padding,
                        opacity: 0.4,
                        filter: "alpha(opacity=40)"
                    },

... it's just not animating the opacity. Any ideas?