How do I locate the contact data for the Address Book in the hard drive on Snow Leopard?
The data is stored in several places. It's best to just export the data to the desktop if you want it in one file.
The complicated answer is in
~/Library/Application Support/AddressBook as well as in various cache files if you have enabled LDAP/open directory integration or other sync options to get contacts into and out of your address book.
If you hit
Address Book Archive you can create a backup. The resulting
Addressbook - <date>.abbu file (which is actually a folder) actually just contains the contents of the abovementioned location.