DNS - Zone NS / delegation in 10.6?

I'm assuming you mean an NS record for a subdomain (creating them for the zone itself is easy in Server Admin -> DNS service -> Zones -> select the zone -> General -> Nameservers). The GUI admin tools don't have a way to delegate subdomains, but you should be able to do it by editing the zone files directly. There are actually two files for each zone, /var/named/db. zonename. (note the period at the end) and /var/named/zones/db. zonename.zone.apple; the second is maintained by the GUI tools, and is not save to edit, but you can add whatever you want to the first one. Just add an NS record to the end of it; it should look something like this:

;THE FOLLOWING INCLUDE WAS ADDED BY SERVER ADMIN. PLEASE DO NOT REMOVE.
$INCLUDE /var/named/zones/db.zonename.zone.apple
subdomain.zonename. IN NS delegatedserver.example.net.

Be sure to include the trailing periods on both the subzone and delegated server portion. Then stop & restart the DNS service to get it to reload the zone, and test to make sure it's actually serving the info properly (it's very picky about its file syntax, and if anything's wrong it tends to skip the zone, or even not start at all).