Circle trailing/fading effect

You know in Firefox when a page is loading, it will display a circular loading animation in the top right corner? See how it has this kind of fading effect – I’ve seen this kind of thing done in Flash before, but cannot work out exactly how. I’d probably be able to do it with tweening/animation, but I’m sure there’s a better way of doing it using ActionScript.

Here are a couple of movies I’ve seen that are kind of similar: – this is kind of the effect I’m after, only not 3D like this – the animation that is occuring behind the preloader with the fading circular effect is what I’m trying to do.

If anyone has seen a tutorial on the web that explains how to do this kinda thing, I’d appreciate the link! Thanks!