restful validation approaches

I'm curious how others who are adopting the REST approach are handling
error messages on creates and updates when submitted via XML for
example.

Are you giving any kind of feedback (ala errors_on) on what the error
is? Or are you just returning some type of http status code? If it is
an http status code, what status codes should be returned for when the
password doesn't match for example?

Thanks,

Steve