Microsoft is learning that people are very reluctant to update their sites to work correctly with IE8. To help rectify this it will be relying on the web community to report sites that have issues with IE8. The report will feed an opt in feature that will automatically update the sites that should render in compatibility mode.
What isn't clear from the post on the IE Blog regarding this new opt in feature is what if any performance impact there is for this new feature. I assume the time that this takes will increase as the number of sites reported as requiring compatibility mode increases.
You will be able to check the number of sites active in the list by placing the following in the address bar - res://iecompat.dll/iecompatdata.xml
The next IE8 build, which should have this feature, should come out sometime in early 2009. It should also be in the next Windows 7 drop, Beta 1, that is guesstimated at Jan of next year.