Have you tested this or are you guessing? In the article there is now a test box using the following code. If !important worked in IE6 then the box would look the same in both browsers. But it does not because IE6 continues to use the last defined statement even though it is being told to use the first via !important.

.testbox {
width:100px;
padding:10px !important;
padding:20px;
border:1px solid #CC0000 !important;
border:5px dotted #CC0000;
}