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.

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$('body').morph({
'background-position':'0 -140px',
'background-position-y':'-140px',
});