Perhaps this article might help?
http://www.evotech.net/blog/2009/03/ie8-css-support/IE8 compliance mode / preventing IE7 rendering in the IE8 browser
Important to know is that IE8 has a button that allows users to render your page in IE7. If that button is clicked enough, your page will always render in IE7 for everyone (not just those who clicked the button). So, do you have to test your pages in IE8 in IE7 mode? No, you don’t. To ensure that your page is not rendered in IE7 mode inside the IE8 browser add the following meta tag to your pages:
<meta http-equiv="X-UA-Compatible" content="IE=8" />Alternatively, if you want your pages to render in IE7 mode you can write: <meta http-equiv="X-UA-Compatible" content="IE=7" />. The issue with this is that IE7 rendering in IE8 is not identical to plain IE7. So, I recommend against using this meta tag.
So, to ensure that you DON’T have to test in IE7, IE8 and IE8 in IE7 mode, add the code above
So my guess would be that adding the tag <meta http-equiv="X-UA-Compatible" content="IE=6" /> to your pages might enforce to only render it in IE6 mode.