A note on Microsoft’s IE blog points out that the feature — designed to ensure sites render as expected — isn’t working everywhere:
People who downloaded Win7 RC “early” may be running without a Compatibilty View list because the list for the Win7 RC is still working its way through the deployment system. Depending on the sites you visit, you may or may not notice its absence. It will be available soon.
You should also remember that you can’t upgrade directly from the Windows 7 beta without a minor hack.