Trusted, not necessarily centralized. Having to trust the price feed is an issue, but having to trust the market which can be manipulated is the issue Bitshares will have.
I don't really see an easy answer. I don't have complete faith in the market or in its participants.
It will always be some blend of a set of centralised feeds, though. It could suffer from a similar problem that banks inflict on bitcoin exchanges who use their services - if all the feed providers somehow decide to stop serving data to mastercoin, there is a big problem.
Cheers, Paul.