Dave, did you mean like this ?:
if (Calendar.is_ie7) {
br.y += window.scrollY;
br.x += window.scrollX;
}
else if (Calendar.is_ie) {
br.y += document.body.scrollTop;
br.x += document.body.scrollLeft;
}
else {
br.y += window.scrollY;
br.x += window.scrollX;
}

If I'm dong it right, this is better, except in IE7 when the positioning element is at the bottom of the screen, then the calendar is also, half off the screen. Seems to be ok in IE6 & FF.
For Elyoukey's fix, this seems to occur in all the above browsers.
We are so close now...