Hi ! I posted this in the comments of the offset page but of course it wasn't the good place :S Sorry for that, here is the post :

Don't know if it's a bug or if I'm just being noob but I found a strange behavior when offset() is used with only one coordinate :

$('element').offset({left : left_coord});
works perfectly in Firefox, Opera, and Safari but throws a javascript error in IE 6, 7 or 8 ("invalid argument").

IE needs both coordinates to be specified, like this :
$('element').offset({top : top_coord , left : left_coord});