I think it would be a bad decision to just delete the default trust list of all the new members of the forum. It would just make no sense for the trust system to newbie while they should be the one's who should be taking the most advantage of it.
Also, in theory, it will be seen who will try to cheat the system. For example, within a couple of minutes 10 accounts are registered, and immediately the same nickname appears in these 10 newbie accounts trust lists, or even a list of users, whom these 10 beginners begin to trust.
Yes, this is a possible crack to the system and it should surely be considered by theymos. A person can created many number of alts to put himself on a default trust list.