lumberg and i had this discussion recently and I'll tell you my own personal interpretation. then you'll probably go search google for a better definition and find I am all wrong....
self-absorbed: living in your own little world. not someone who "puts themselve out there" so to speak. like i am satisified with my life and i require very little in the way of other people's involvement in order to be happy in my life.
self-centered: thinks the world revolves around them and their needs/demands. puts themselves before others and expects others to adhere to their demands. inflated sense of self-importance, so to speak.
so to me, these are two totally different characteristics.