The only way I found, unless your content is a static-height, was to do it with jquery like so:
$(document).ready(function(){
inner_height = $('#inner-div').height();
outer_height = $('#outer-div').height();
margin_calc = (outer_height - inner_height)
$('#inner-div').css('margin-top', (margin_calc+'px'));
});
This will work for a column-div within a containing-div, if another column's height is larger.
It is unbelievable this simple thing is not 'built in' via a css-property (i.e. "float: bottom"), that doesn't break everything else with absolutes, etc.