Explains it all: http://jsfiddle.net/7d3KZ/
This was working before 1.45.
As far as I can tell, the IE5-7 opacity filter is overriding inline style.
Seem related to #2274
Ok. Found a solution. It's a combination of using the right CSS rules for opacity and seems that the filter style needs to be re-set doing box.setStyle('filter', ';');
I tested http://jsfiddle.net/7d3KZ/6/ in IE 7 and 8 standalone.