clearfix classes don't work in IE versions < 8

The clearfix classes in default.css don't seem to clear floats properly in IE versions prior to 8, since those versions of IE doesn't recognize the :after pseudo-class.

In order to get my layout to work, I had to add <div class="clear"></div> to my layout at certain points, and remove the "clearfix". From the referenced article, it looks like it would be necessary to add

.clearfix { zoom: 1; } to an IE-specific stylesheet.

Please correct me if I am wrong about this - I am not a CSS expert. I could attach my page.tpl.php & a link to the site in question if it helps clarify this issue.