They are doing this by allowing the web-site itself to control which rendering engine IE8 will use when you browse. This will mean if you site is already compatible with IE7, it can also be compatible with IE8 by simply including one line of HTML in the header...
<meta http-equiv="X-UA-Compatible" content="IE=7" />
This will apparently force newer version of IE8 to render the current page in IE7's format, which from a standpoint on the priority of getting things upgraded, can allow it to go a few notches down on the list rather than being the number one.
For once it will be nice to worry about feature sets, improvements and bug fixing rather than worrying if the styling will still work when the next version of IE comes out.
Thank you Microsoft IE team!