Steve Kent agreed to follow up with RSA Labs on this topic. He notes that interpretation of this field differs based on whether the signature mechanism is being used for non-repudiation or authentication and integrity. For non-repudiation, in the absence of the privateKeyUsageValidityPeriod extension, then the private key is assumed to have been valid during the validityInterval, but the public key is assumed to be valid forever, i.e., for use in verifying signatures generated during the validityInterval timeframe. Note that for non-repudiation to be effective, one must establish the timeframe in which a signature was purportedly applied, and that the key used was not reported as revoked during that time frame. Others note that the definition of this field applies to the certificate, not to either the private or public key, but to the binding between the public key and the other certificate attributes.
Work is independent of signature and hash algorithms and of underlying transport mechanisms.
NIST also wants to develop CA, RA, and client reference implementations, for free distribution. These implementations should support multiple algorithms and provide a sanity check for the test suite. NIST is looking for assistance in developing the reference implementations. Denis comments are already available via the mail list, and so are not reproduced here.