OK so I've got an IE problem - surprise! When I turn supercache on, the sidebar disappears but only in IE and only on pages that also have comments.
To make sure it wasn't a widget conflict, I removed the widget code and set it to display some text. Still no love. The same sidebar displays fine in IE on pages that don't have comments enabled.
If I disable comments for the page, the sidebar returns. So apparently they don't like each other.
Any ideas on making this work? I have to leave Supercache off for now. :(