Thanks to Lyndon Higgins for the following:
Microsoft will be releasing Internet Explorer 8 to display content in its most standards compliant way by default. Enabling Internet Explorer 8 to display web content in its most standards-compliant way by default is the right way to help move the Web forward.
However, browsing with this default setting may cause content written for previous versions of Internet Explorer to display differently than intended. This creates a call to action for site owners to ensure their content will continue to display seamlessly in Internet Explorer 8. In order to maintain backwards compatibility with Internet Explorer 7, we have provided a compatibility meta-tag usable on a per-page or per-site level. Adding this tag instructs Internet Explorer 8 to display content like it did in Internet Explorer 7, without requiring any additional changes.
We are encouraging site administrators to get their sites ready now for broad adoption of Internet Explorer 8, as there will be a beta release in the third quarter of this calendar year targeted for all consumers.
When IE8 goes live many websites will still not be completely standards compliant. Site owners who follow current developments will be ready but most websites, such as small business/private, will not be updated within this time frame resulting in many websites not rendering well in ie8.
Im not sure where the basis for "many websites" not rendering well in ie8. I don't have any precise figures on current site compatibility.
Furthermore the default mode of ie8 is standards based, but other modes can be invoked.
Internet Explorer 8's built-in Compatibility View button enables you to display websites that were designed for older browsers
See also readiness toolkit: