Rosetta is a software technology that allows Intel-based Macs to run most, but not all, of native PowerPC applications. This How-to explains some details of using, enabling, disabling and status checking Rosetta.
The first thing you need to know is that Rosetta is a very small piece of code, about 2.1 MB. It's not at all like the Classic environment of old, so any obsession with eradicating it from your system is unwarranted. It just waits in stand-by mode and is only invoked when a native PPC application is launched.
Second, if you didn't have it loaded in Leopard, the Snow Leopard upgrade won't install it by default. What happens is, when the OS sees that you're trying to launch a native PPC app, it will open a dialog box and invite you to install Rosetta -- even if it's already installed. (You'll need a network connection for that.) If you aren't on the Internet, you can use the Snow Leopard DVD to install manually. Our Jeff Gamet explained the complete installation back in early September.
Dialog box for install of Rosetta
So that's one reason why it's a waste of time to try to delete a functioning install of Rosetta. Some day, you may need it for just a few minutes, so once you have it, just let it lie dormant, doing nothing most of the time, and taking up about a much disk space as a two minute MP3.
How can you tell if an app is native PowerPC only? Select the app and do a Get Info (CMD + i). If it's PPC only, the Kind field will say, simply, "Application."
The Three Kinds of apps
That's in contrast to: Application (Universal) which has code for both. Or Application (Intel) which is Intel only.
Removing Rosetta requires some UNIX expertise, and, as I said above, it isn't really worth it. It's just a microscopic piece of code that's utilized by the OS only when needed.
I have seen some websites that provide a command line sequence to enable and disable Rosetta. If you disable Rosetta, the only practical outcome is that when you do launch a native PPC app, you'll be notified with the dialog box above, and the system will want to download and install Rosetta all over again. This can only be useful if you really, really need to know that an app you've launched is native PPC and want to set about the business of replacing it with a more modern version, say, a Universal app. But you could have checked that, if in doubt, with the Get Info function as well before launching.
For the sake of completeness, here's the command to disable Rosetta:
$ sudo sysctl -w kern.exec.archhandler.powerpc=/usr/libexec/oah/RosettaNonGrata
You'll need to be logged on as administrator because the Administrator of the Mac is in the "sudoers list" (/etc/sudoers) and can manage files owned by root.
Here's the command to enable Rosetta:
$ sudo sysctl -w kern.exec.archhandler.powerpc=/usr/libexec/oah/translate
To determine the current status of your Rosetta, enabled or disabled, enter this on the command line:
$ sudo sysctl kern.exec.archhandler.powerpc
If Rosetta is enabled, you'll see a line that includes "translate." If not, then the line will include "RosettaNonGrata."
Unless you have a specific, expert need to disable Rosetta, it can safely be left "enabled."
My advice is that once you have Rosetta installed, just forget about it. Don't try to delete it or disable it. That is, unless you really, really need to be alerted when a native PPC app tries to launch. Mostly, Mac IT managers will watch for that, not casual users at home.
Apple is all about gentle, methodical translations. At some point in the future, Rosetta won't even be included in the OS, or be available, and it will quietly disappear, just as the Classic environment has. By now, you should have identified any critical apps that are PPC only and found an upgrade or substitute. That's a more important task than a false sense of aesthetics for those who just don't want Rosetta on their Mac anymore.
Rosetta, by the way, is not a panacea. It translates G3, G4 and AltiVec instructions into Intel native code, but not any special, G5 only instructions. There are other software components that can't be handled by Rosetta, for example screen savers and kernel extensions. See this article for more background information. Fortunately, as part of your migration over the years, any software that had those limitations has been upgrade or removed.
In summary, if you never launch a native PPC app, you'll never encounter Rosetta. But if it does get installed, just leave it there. It'll quietly do its job for you when needed. As Apple says, it's "The most amazing software you'll never see."