From the draft minutes:
CERT Draft Discussion - Paul Hoffman
4) Attribute Certificates: Paul noted that it is an action item for the
PKIX WG to define the use of ACs. David Solo proposed that all references
to ACs should be deleted from all of the S/MIME specifications until the
syntax and use of ACs are well defined. Russ and John Pawling made the
point that X.509 defines the AC syntax. However, Paul and Dave strongly
believed that ACs should be deleted from the S/MIME specs since their use is
not defined and the PKIX AC syntax may be changed. Nobody objected to this
proposal. The details of this change will be discussed further on the
S/MIME list.
Okay, let's have the discussion....
Russ