Web sites still not fully compatible with IE 8.

The bad news: More than 2,000 highly-trafficked Web sites are still not compatible with Internet Explorer 8 and require IE’s Compatibility View support to display properly. The good news: That is down from about 3,100 sites a year ago.

The new data is courtesy of a March 2 post on the Microsoft IE blog.

A year ago, when Microsoft first released Internet Explorer (IE) 8, it got kudos for making its browser somewhat more standards-compliant than previous versions. But increasing standards compatibility meant more incompatibility, as many Web-site developers had tweaked their sites to take advantage of far less standards-friendly IE 6 and IE7 and weren’t in any rush to make changes to accommodate IE 8.

Source -
Microsoft: More than 2,000 major Web sites still not fully compatible with IE 8 | All about Microsoft | ZDNet.com

That's slightly misleading, some sites (like ours) just have IE compatibility mode enabled not because they are incompatible with IE8, but just so there are less browsers to test against.