Run the sample code stated here on the Emulator. If you see Circle looks normal with out the Opacity set, then the bug is reproduced.

var circle = Circle {
    centerX: 100 centerY: 100
    fill: Color.YELLOW
    radius: 25
    opacity: 0.5
}

var appFrame: Stage = Stage {
            x: 400
            y: 400
            width: 250
            height: 250
            title: "Text Test"
            //style: StageStyle.TRANSPARENT
            //style: StageStyle.UNDECORATED
                scene: javafx.scene.Scene {
fill: Color.BROWN
             content: [circle]
}
            visible: true
 }

FX Build: 1.0.1 b02 Build
OS: Win XP