How does the Windows 7 installer know which version of Windows to install if you don't put in a key during install?
I assume Windows 7 will work the same way Vista did - it asks you. If you choose Ultimate, it installs Ultimate. If you choose Business, it installs Business. If you're foolish enough to choose Ultimate and then you go out and buy one of the Home edition license keys, you'll end up having to reinstall with the appropriate version in order to activate it.