ProKit 6.0.1 has been breaking Final Cut Studio for some people and so far there has been no fix from Apple.
Here is how to restore an older version:
1. You will need to obtain a copy of the old ProKit.framework file either from a friend, another computer on the network or a Time Machine backup.
Update: I have discovered that the version of ProKit 5.1 on Apple's site that is supposedly Leopard-only is also Snow Leopard-compatible. You may need to install ProKit 5.0 first. Before installing, open up the Terminal in /Applications/Utilities and type sudo pkgutil --forget com.apple.pkg.ProAppRuntime, then enter your admin password.
2. In the Finder, navigate to /System/Library/PrivateFrameworks and copy the ProKit.framework file to your desktop.
3. Ctrl-click on the file on the desktop and select Compress "ProKit.Framework".
4. Delete the original file from your desktop and empty the trash. Leave the compressed file there for now.
5. Copy the older ProKit.framework file to /System/Library/PrivateFrameworks, overwriting the newer one. You will most likely need to enter an administrator password to do this.
6. Final Cut Studio applications should now work and it should be safe to delete the compressed file. If they do not, try restarting your computer.
7. If something goes wrong, decompress the compressed file and copy it back to the PrivateFrameworks directory.
(The reason for compressing the file is to ensure that OS X doesn't continue to recognize it as a framework and potentially try to link to it.)