It appears Lightbox has a bug in IE when the screen height exceeds 65,505 pixels.
I have created two test pages for you to see. One that shows the bug and another that shows it working. The problem seems to be ONLY with Internet Explorer (of any version).
In the following test htm pages click the "Click Here" link at the top left corner. You will see a lightbox pop up on screen. The issue is related to the opacity level. When the screen height is <= 65,505 pixels the opacity works fine for IE. But if the height >=65,506 it fails. It is not an IE bug because i tried using filter: alpha(opacity) and it works fine. It seems to be a lightbox bug.
Test 1: Opacity Works because Height of Screen is 65,505 pixels: http://fatcatdaily.com/test-works.htm
Test 2: Opacity Fails because Height of Screen is 65,506 pixels: http://fatcatdaily.com/test-fail.htm
NOTE: You MUST be using Internet Explorer to see the BUG