From: http://www.macosxhints.com/article.php?story=2009101505134452Ma
If you have already installed Rosetta on your Intel Mac in Snow Leopard, you may want to disable it, so as to prevent the system from running PowerPC apps without informing you that they are non-native to your system's architecture. The reason for disabling Rosetta is not so much to save space (as it's only 2.1MB), but more about keeping your system running native apps for higher performance.
To disable Rosetta, simply run the following command in Terminal:sudo sysctl -w kern.exec.archhandler.powerpc=/usr/libexec/oah/RosettaNonGrata
To re-enable Rosetta after it's been disabled, just run the following command in Terminal:sudo sysctl -w kern.exec.archhandler.powerpc=/usr/libexec/oah/translate
These commands set a system environment variable that tells the system which program to run when you try to launch a non-native app: to ask you to install Rosetta, or to run it using Rosetta.