S/MIMEv3 requires certificates to be optionally looked up by Subject Key ID. Currently, there is no way to do this in NSS. The cert database format is not up to this task; though there is a record containing subject key ids, it needs to be looked up by subject DN (which you would not know at this point) - so in the end, one would have to traverse all the subject records and match the subject key id. However, the necessary attributes for a PKCS#11 style C_FindObjects lookup must be defined and implemented first.
Changed the QA contact to Bishakha.
This has been fixed for a while. CMS has no problem looking up certs by SubjectKeyID. pk11cert.c rev 1.110 Dec 17, 2002 has part of the change. NSS 3.7 had rev 1.109 NSS 3.8 had rev 1.116, so I've set the target to 3.8 to reflect when the fix went in. (For Mozilla users, that would be Mozilla 1.4a Release.)
arg, I meant to mark this fixed