While I have not noticed this to be a problem myself, I can appreciate your concern.  To my knowledge, that behavior has little to no change since IE6.  If you want IE to always call for the most recent version of the page, you can change your settings in the browser, but you're right.  The default behavior is to not do this.  The thing that makes IE call for a fressh download of the web page and its associated file is the HTTP Header that describes the date/time that the page was last updated.  I remember seeing once that DNN handles this for you, by automatically adjusting that value in the META tags.  You can easily check this by viewing the rendered source of your site.

<meta name="expires" content="[datetime]" />