I’m trying to do a fully customized Facebook “Like-Box”. I want to have this fixed width div and be able to slide left or right to show more faces (I would prefer ajax, but that doesn’t seem possible, or does it?).
If there’s no ajax way to LOAD more faces, I would like to pre-load, let’s say, 50 faces, on this small div and press left or right to slide more content (faces) from inside the div.

You can do this in javascript by setting the scrollLeft property of the div. Or if using jQuery, you can use the scrollLeft method.
IE:
$("#myDiv").scrollLeft( 250 );jQuery API: http://api.jquery.com/scrollLeft/