Different rotateZ behavior in IE

[raytri]

I've written a pair of functions that rotate a div back and forth around its center point:

 function graphic_rotate_cw() {
    jQuery("#bg-circles").animate( {
        rotateZ: '+='+.15
    },4000,function(){graphic_rotate_ccw();});
 }

function graphic_rotate_ccw() {
    jQuery("#bg-circles").animate( {
        rotateZ: '-='+.15
    },4000,function(){graphic_rotate_cw();});
}

They work exactly as expected in FF, Chrome and Safari. But in IE7-9, the image rotates around its lower-left corner instead.