The application itself is a universal binary, but the installer requires Rosetta. It auto-installs Rosetta when you run the Office installer, unless you installed using Terminal (http://www.mactalk.com.au/11/72330-install-microsoft-office-...)

If you didn't use Terminal to install Office, then you must have either upgraded Snow Leopard instead of doing a clean install, which would allow you to use Office without running the installer again, or you have Rosetta installed without realizing it.