The answer to this really lies in the capabilities of CSS. .pull-right and .pull-left both apply a micro-clearfix and then float the element left or right in Twitter Bootstrap.
Creating .pull-top and .pull-bottom class' would not be feasible as the parent elements would need to have a position:relative applied, the element would then need man-handled around with top and bottom properties to have it placed properly. The alternatives would require the element to have a fixed height with negative margin applied to the top, of half the height and top:50% applied. Again this would need the parent to be position:relative.
As with any CSS framework, there are just some things you will have to do yourself
a example of Pull down bootstarp:-
$('.pull-down').each(function() {
var $this=$(this);
$this.css('margin-top', $this.parent().height()-$this.height())
});