Please correct me if I'm wrong but:
I would guess (educated guess) that you cannot index on uniqueMember because it is a multi valued attribute. Whereas, uid and cn are only single valued attributes (only one uid and cn are allowed within an entry).
You are wrong. You can index multivalued attributes just fine.
--Quanah
-- Quanah Gibson-Mount Principal Software Developer ITSS/TSS/Computing Systems ITSS/TSS/Infrastructure Operations Stanford University GnuPG Public Key: http://www.stanford.edu/~quanah/pgp.html