I tried to make a brushed surface of realistic metal like the apples iCloud icon. (see attached file).

With the technique described at http://ryanler.wordpress.com/2007/05/31 ... in-effect/ I get a horizontal or vertical metal effect. Is there a way to create this effect in a circular way?

Welcome to InkscapeForum!

I don't know of any tutorials, which doesn't mean there isn't one somewhere (or more). I managed to fake a cone- or wedge-shaped gradient like in the image in this message: http://img38.imageshack.us/img38/4652/welcome12d.gif using the Interpolation extension. You might be able to fake those wedge-shaped highlights using Interpolation. Actually, just looking at your sample image, I think most of it could be done with interpolation -- the concentric circles as well as the highlights. (I'm almost tempted to give it a try, lol!)