CSS: Relatively Centering Absolutely Positioned Elements
Yup... you you can do it so long as you're willing to do a little light math--and you can even do it in a somewhat responsive way. Here's a quick overview of the various ways to center an element with absolute positioning.
Background: CSS Centering
Centering and CSS have always been a challenge; I don't think this all that contentious a claim. If you think about it for more than a second, though, you can imagine why this might be the case: center is a relative term--it can only be defined within the context of another element. In order to center something, then, you have to establish a relationship.