Lorelei gave me an idea in this thread
http://www.v7n.com/forums/showthread.php?t=24102
select, Atom's favorit tool, the pen tool and draw your shape (or convert text into a path, or make a selection an convert into a path), open the path pane.
Select a big round soft brush, set opacity low, like 20%, and select you base color.
In the Path panel click the circle icon once.
Select a smaller brush, maybe change opacity to 10%, and hit the circle icon in the path pannel again.
Keep going till you get the result you want.