I am experiencing and issue only in IE. In FF, Safari, and Chrome, behavior is as expected.
After running the IE8 dev tools profile on the pages, all signs point to the jQuery .css() function, although the details in the call stack make it impossible to find the culprit. I get no filename or line number whatsoever.
The site is http://www.boulevard.com
Thanks!
OK, this might be helpful for others.
In order to see the line numbers and URLs in the IE8 Developer Tools profiling section, you need to enable script debugging in the IE options (Tools->Options->Advanced). Once enabled, the profile will show the file and line numbers in the profile, allowing you to trace these types of issues.
In this case it was the jQuery Colorbox Plugin. in the Colorbox css file, Colorbox specifies the path to some image files and loads them with the IE filter AlphaImageLoader. Unlike the rest of CSS, the paths to these files are relative to the document, not the CSS.
Changing these paths fixes the issue.