how about this for an answer, got it from ‘CSS Cookbook’
CSS to get the effect is:
width:700px; /*total width of left and maincontent divs*/
float: left; /*floats it to the left of the page – leaving room for the right column */
float: right; /*floated to the right of the left column (but will still position to the left of the rightmost column /*
So basically this method works by enclosing the left and maincontent divs in an additional div which is then floated to the left.
hope that helps.