go thru all parents... like this (works on IE as well as on NN):

function _elementTop(el)
{
var et = 0;
while (el)
{
et += el.offsetTop;
el = el.offsetParent;
}
return et;
}

function _elementLeft(el)
{
var et = 0;
while (el)
{
et += el.offsetLeft;
el = el.offsetParent;
}
return et;
}