If you try to set the CSS-property
background-position in Internet Explorer (I tested with 7) and even use that with Mootools' morph/tween-functions, it will not work.
You can workaround that issue, if you set also background-position-x and background-position-y (actually ignored by my Firefox 3.0).
This is the code, I use at koala to move the background upwards.
$('body').morph({
'background-position':'0 -140px',
'background-position-y':'-140px',
});
