See rpc_server/srv_samr_nt.c in function _samr_query_dom_info. The two info levels that return the domain sequence number, 2 and 8, call time(NULL).
John, when you are looking for a "real" sequence-number while using pdb_ldap with OpenLDAP you might have a look at: http://websvn.samba.org/cgi-bin/viewcvs.cgi?rev=5175&view=rev
JRA asked for this bugzilla to be opened during our presentation at SambaXP.
This was fixed a long time ago with the change to generate a seqnum from OpenLdap syncrepl contextCSN. Not sure if this is the right way to close this to get it out of the listing of "My Bugs" in bugzilla.