Tried applying the border to tbody as well as td and tr, but always had the same result with the cell dividers showing.
Finally went ahead and just added an empty row like you suggested, and set the height and background color, and it works pretty well; side borders seem a little off (because the 1px table border is applied to the red section since it's a row), but it's much better than having those dividers show.
Still don't understand why those dividers were showing. I've used a border-top on tables many times and have never seen that happen before. Wonder if it's an IE9 issue.
Anyway, thanks a bunch!