>>I'm not saying that error doesn't/can't occur, but it is true that most universities, fed/state/local governments, and many businesses use SSN as unique key without serious problem. It generally works fine, but you should definitely build in a check to verify uniqueness before you add a new SSN to your DB.
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>One other thing to consider here when using SSN as a key. If there are no tax implications, you can refuse to give your SSN. I know several people who do this. If this happens, you need someway to generate a bogus key.
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Yes -- universities do exactly this, since they get foreign students without SSNs.