How do I restore the default certificate settings in Snow Leopard?
You want the two files ending in *.keychain found in /System/Library/Keychains (note that they may differ depending on your region, I'm not sure if things change depending on your geo-location). 

Unfortunately, there is no way to roll back unless you have a Time Machine backup, or the install files from the OS (then you can extract them using something like Pacifist).

Alternatively, you can run Keychain First Aid from the Keychain Access menu and hope it fixes things but those likely will only deal with errors or anomalies in your user's keychain, not the system's.

ATTENTION: For those in a similar position, these are all the files located under /System/Library/Keychains from a default installation of Snow Leopard 10.6.8 (in North America).