Note the change in IE8 Beta 2 (or sooner)
On the IE Blog, they announced a slight change with the META Tag. I've included an excerpt here:
In IE8 Beta 1, that option is the “IE=7” X-UA-Compatible tag, which instructs IE8 to display content in IE7 Standards mode. However, the scenario this doesn’t address is when IE=7 is applied as an HTTP header to a site that contains Quirks mode pages. The IE=7 HTTP header will force all pages – both Quirks and Standards – to display in IE7 Standards mode. Developers using this header while updating their sites would then have to add the “IE=5” <META> tag to each page they want to keep in Quirks mode. This logic is fine for many websites. However, if a site has lots of Quirks mode pages, or for the case where pages with frames host a mix of Strict and Quirks mode content – as brought to light by IE8 Beta 1 user feedback – the compatibility opt-out adds a bit more work than we intended.
In response to the great IE8 Beta 1 feedback we’ve received so far, we are introducing the “IE=EmulateIE7” tag to address this problem. EmulateIE7 tells IE8 to display standards DOCTYPEs in IE7 Standards mode, and Quirks DOCTYPEs in Quirks mode. We believe this will be the preferred IE7 compatibility mode for most cases. Support for IE=EmulateIE7 is available now as part of the IE June Security Update for IE8 Beta 1 . Installing this update will enable you to verify you’ve applied the EmulateIE7 tag to your site correctly.
See the IE Blog for more details: Introducing IE=EmulateIE7