The Secretary of State and Notary training courses will say that "satisfactory" evidence is Proper ID but not solely a credible witness under oath. The notary must get satisfactory evidence AND an oath. PEPSI P-hoto E-xpiration date P-hysical description (except on Passport) S-ignature I-ID# I-ssue date (within 5 years) which boils down to a DL current or <5 years old; Out of State DL current or <5 years old; a Passport (including other countries); Military ID; Government issue ID; Not a Green Card; Other(?) picture ID, issued by a government (authority) and dated. BUT BE AWARE this is MERELY satisfactory evidence (which gives YOU an affirmative defense if there is a criminal (or legal) consequence. WHY DO I SAY THAT? Well, I worked for the data processing element of Orange County criminal justice and there were "active" people with more than 175 names (many of which had the ID required above). They had state issued ID and credit cards and even Social Security cards .. 75 or 100 were not unusual (they'd be found and confiscated pursuant to a search warrant). Well that person could have presented ANY of their 100 or more ID's and how could a notary determine which is valid? (the police finger print them and run the prints; then 175 AKA's come back .. and everybody has a good laugh). SO WHAT YOU ARE REALLY DOING is swearing that YOU did your job and verified the identity to the best of your ability by PEPSI and a sworn oath of the signer. (You cannot run the prints, but you did get a thumb print). The problem gets worse as personal computers get better. If someone has 100 or more ID's 99 of them must be counterfeit. New California law holds that a credible witness (or witnesses) cannot be the sole identity of the signatory (and now you can see why). Power of attorney cannot be authenticated by a subscribing witness. It also prohibits the use of a subscribing witness for any documents requiring a notary public to obtain a thumbprint from the principal in the notary public journal. (because they can run the thumbprint if it is a criminal matter). Today (and from now on) identity theft (involving hundreds of thousands of dollars) is overwhelming and world-wide. Just about anything can be counterfeited. YOU ARE PROTECTING YOURSELF but you cannot actually know if the signer's ID is phony.
Of course Mr. Pitt's post only applies to California. I notice on page 8 and 9 of the 2011 California Notary Handbook that the oath of a single credible witness IS satisfactory evidence PROVIDED the notary personally knows the credible witness AND check's the credible witness's paper ID documents. Alternatively two credible witnesses with satisfactory paper ID who the notary does not know may be used. In any credible witness scenario, the witness must swear that the "credible witness reasonably believes that the circumstances of the signer aresuch that it would be very difficult or impossible for the signer to obtain another form of identification". So, yes, paper ID is required in the credible witness scenario, but not from the signer, only from the witnesses.
Subscribing witnesses are a different matter altogether. They are for cases where the signer can't come before the notary, so the subscribing witnesses swear that they saw the signer sign, or that the signer acknowledged signing. This, of course, is weaker than having a notary take the acknowledgement, so understandably California does not allow it for some important documents. My state goes even farther; only a judge may prove execution by subscribing witnesses.
The notary handbook in my state, Vermont, essentially reverses the preferences for the method of identifying the signer; personal knowledge by the notary is best, identification by one credible witness known to the notary is second-best, and paper ID is a distant third.