With RSA Public/Private key technology everybody knows the public key and uses it to encrypt messages that can only be decrypted with the private key. Likewise, you can authenticate a message by decrypting with the public key what can only be properly encrypted using the private key. So, the private key is never shared with anyone such as in DES style of encryptions. The one other entity that knows your private key (disregarding fraud and mistakes) is the escrow agent(s) (which may include the US government) that assigns public/private keys. Presuming the escrow agent is "trustworthy" and the enduser doesn't expose their password it isn't "possible" to forge documents that use this technology.
Hence, the critical security issue becomes the key management and distribution issues by the organizations themselves and the Certification Authorities that assign the keys. since there is now at least a minimally sufficient technological solution available worldwide including DES/Kerberos authentication and RSA Digital Signature technology-- as you point out in your message. The MCC EINet project that I am a member of is one of the groups working this issue.
Regards, -Bruce