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

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?