That was only for quirks mode, where IE6 acts as if it were IE5. Anyone trying to support IE5 is a glutton for punishment, who should be given a wide berth by those not into the S&M scene.
IE6 takes the padding into account in the 100% absolute width of your bottombar and stretches the parent.
Also reduce the margin/padding on html to allow the lightbox to center properly. Add padding/margin to the thumbnails themselves to move them away from the edge.
Use this:
body{margin:0;padding:0}/* fixes central position offset.
* html #ui-lightbox-bottombar {
padding:5px 0;
margin:0;
}
I suspect, not tested, that the offset is taken not from the body edge, but from the right edge of the thumbnails, and that it is not a doubling, but is 100px apparent added offset.
I’m not enamored of the light box paradigm, so have little info as to its methods, especially as implemented with jQuery libraries.
Sorry I can’t be of more help, but that might give you a new clue toward debugging.