The Rosetta dynamic translator that lets PowerPC apps run on an Intel-based Mac is optional and not installed by default on Snow Leopard. While there are Intel versions of most applications available today, for people like me who still have PowerPC utilities it was neat to see Apple make Rosetta dynamically available for download on Macs that do not have it installed but need to run older code. Despite it's availability and ease of installation, at this point I would consider Rosetta to be Apple's last effort at providing PowerPC support, and a feature that is not guaranteed to work.
For some people who have included Rosetta in their installations, PowerPC applications may crash or refuse to open when they are launched. In some instances this is a conflict between Rosetta and the code being run, but in others there are a few things you can do to try correct problems with PowerPC applications.
1. Clear application preferences
Clear the preferences for the program and try again. Locate these files in either the /username/Library/Preferences/ folder, or the /Macintosh HD/Library/Preferences/ folder and remove them for the program that is having difficulties. The can technically be called anything, but by default they have a name similar to "com.apple.Mail.plist" (for Apple's Mail program).
2. Remove the "ByHost" preferences
In addition to normal preference files, the system contains machine-specific application preferences so the same boot drive can be used on various hardware and not run into problems with either system settings or applications running on it. These are stored in the "ByHost" folder in the "Preferences" folder, so go there and try to find relevant preference files for the applications you are using.
NOTE: Preference files are easily remade, so you can empty these folders and have your applications and system rebuild all preference files, which may be another thing to try. If you do this, be sure to keep the preference files in a location such as your Desktop so you can replace ones you need (restoring any lost settings).
3. Update the dyld cache
Updating the dynamic link shared cache is the replacement for the old "update_prebindings" routine that would help clear slowdowns and other problems with newly installed applications. Running the following command in the Terminal should ensure the dyld cache is set up properly:
update_dyld_shared_cache -root / -force
This normally should not need to be run independently, but if a system component such as Rosetta is not loading correctly and is hanging or crashing, then manually updating the cache can help.