Thanks, Andrew, for working through this issue w/ me. Seems that so far we've learned two things. The white space right-side padding issue is happening only when (1) using IE, and (2) when the Reports Module w/ HTML Template Visualizer is in the Top Pane.
When using Firefox 2, the right-side padding issue disappears. When moving the Reports Module to a pane OTHER than Top Pane, the padding issue disappears.
Meanwhile, I plan to 'workaround' by redesigning my page, moving the Reports Module w/ HTML Template Visualizer outside of the Top Pane.