I have a problem with Lightbox not displaying properly in IE. The lightbox ends at a certain point (usually after the part that reads "Photography").

My site is:

http://www.liesldexheimerdesign.com/portfolio.html

I'm not sure if it's a problem with CSS or HTML...

Yeah its a problem in your style sheet. Just go through the style sheet again where you used the light box.

The problem is that you've used position:absolute for everything - and IE has a huge problem with that, very buggy.

Learn to use the normal document flow and floats and you'll have better luck.
You've got serious case of div-itis going on there - you've really over-complicated that layout, but I suspect, from the look of it, that you exported from Photoshop to get that code.

There are easier ways to accomplish your layout with far fewer divs, try reading this: www.cssslicingguide.com