Currently, Ripple Labs runs five validators and most Ripple servers have only these five validators on their UNL. A transition is in the works to distributed community management of UNLs, and that transition is expected to be completed during the first half of 2014.
In the final scheme, publishers will be able to publish lists of validators (by public key). They will ensure validators accurately report their identity, jurisdiction, type of organization, and so on. Publishers will monitor the validators they publish to ensure they are operating reliably. Since validators must sign every proposal and validation, most types of misconduct will be easily provable.
The Ripple server software will come with a default list of publishers, though you can add or remove them as you see fit. You can also manually add validators or remove validators. You will be able to tune the policy based on the parameters the publishers vouch for. For example, if you don't want any validators operated by governments, you can opt to do that. If you don't want any validators in Australia, you can opt to do that.
Your server will periodically poll the publishers and statistically combine their lists to come up with a set of validators to use.