Never let it be said that Apple’s Open Directory command lines tools make complete sense. After all, why would you think to use the new dsmemberutil tool in Leopard to find the Universally Unique Identifier (UUID) for a user account?
You would have if you had read the first line of its man page, maybe, where it outlines exactly how miscellaneous — and I say that in the kindest possible way — the command is: “dsmemberutil — various operations for the membership APIs.”
Enough lead in; the UUID for a user (as well as a group) can be found with this tool:
$ dsmemberutil getuuid -u 501
E4BDC9C9-AD89-4CD6-BE25-16B197669B47
Choose a user with either -u with the userid, or -U with the short username.
One way this can be exceedingly useful is if you want to track down the shadow password file corresponding to a specific user account:
$ sudo ls -1 /private/var/db/shadow/hash | grep `dsmemberutil getuuid -u 501`
E4BDC9C9-AD89-4CD6-BE25-16B197669B47
E4BDC9C9-AD89-4CD6-BE25-16B197669B47.state
Hey, look, the UUIDs match, saving a trip to dscl!