clearfix not sufficient on main-inner for IE7

I created a 3 column layout and I'm finding with IE7 that the center column (main-inner) is getting over-layed on the first column (left sidebar), though of course things look fine in FireFox. In looking at the HTML generated by the Zen Classic theme for comparison, I see that the "system provided" clear-box class is being used for the main-inner content, when I changed page.tpl.php for Airy Blue to use the clear-box class rather than the clearfix class, my IE7 overlay issue went away.

Thanks for a nice looking theme!
--Scott