Problem saving nested attributes for has_many associations (Rails 2.3 RC2 and Edge)

I've tested this on both 2.3 RC2 and Edge. This pastie contains
model, helper and view:

When I save a vocabulary - its terms don't save. I'm fairly sure it
worked in RC1. Would appreciate any insight.

I just realized that :reject_if was causing the problem. What
happened to it in latest rails? Could anyone explain?