The problem is that the width of the tabbed panel is not spanning 100% of the center column. It displays fine in Firefox.
The layout is the "liquid, 3-column liquid, header and footer" supplied by Dreamweaver CS3. The 3 columns are encompassed by a container div set to a width of 80%. I added a min-width of 800px. The middle column is position: relative. It has a left and right margin of 23.5% and 24% respectively.
I notice that the width of the tabbed panel in a fully maximized IE7 browser window only extends as far as when the browser window is reduced to the "min-width" of 800px.
In other words, the tabbed panel is behaving as if the layout is at min-width all the time.