Constraint, Rule, or Trigger?

I've had experience with SQL and database design, but not much with rules &
triggers. What's the best method to accomplish the following it PGSQL:

_Table A_ has a boolean field 'status'

_Table B_ has a 1-1 child relationship to Table A.

The rule, trigger, or contraint would have to enforce that: no record could
be created in B  if  the 'status' field was false in the parent record in A

Just point me in the right direction.
Thanks


-- 
Randy Perry
sysTame
Mac Consulting/Sales