Why can't I add a string containing a script tag to innerHTML in IE

I'm trying to do the following (I'm using the prototype library):

var div = document.createElement('div');
div.innerHTML = '<script src="somescript.js"></script>';
$('banner').insert(div);

In IE, div.innerHTML property is always equal to "" after I set the property in the second line.

This snippet is inside a function which is overriding document.write() in an external vendor script, so that is why I am doing it this way rather than creating a script element and appending it to the div element directly.

Any help would really be appreciated, this is giving me grey hairs!