IIS7 and OutputCache

I have a site hosted on two servers IIS5 (for development in XP) and IIS6
(for production). The application code is the same on both and both servers
handle output caching directives the same. However I noticed a troubling
change when I switched my dev machine to Vista and IIS7. Even though I
specificaly disable caching in every page with the following directive,

<%@ OutputCache Duration="1" VaryByParam="None" Location="None"%>

Modal browser windows also known as a dialog popups are caching their
content on any second request until the browser session that created it is
closed and reopened. Again this problem does not exibit on non modal popups
nor on previous versions of IIS. What has changed and how can I disable
caching for these pages?

Thanks
Perry