Just to complete your answer, Jinx, the wastegate does regulate the boost pressure. But, it does so by letting exhaust gas go around the turbo instead of through through it. The wastegate happens completely on the exhaust side, while the bov is strictly on the fresh air side.

To hit Tnathe's math problem... The bov has a spring that holds it closed with a force of around 8psi worth (assumption, approx). When you're boosting 12psi (or 20 or whatever) the valve sees 12psi on the bottom, but also 12psi on the top because of the vacuum hose to the intake manifold. So, with those pressures equal, the bov is held closed by the force of the spring. When you close the throttle, the intake manifold hits vacuum of around -10 psig. So now, you have 12psi pushing up against bov, 8psi worth of spring pushing back, and now 10psi of vacuum pulling against it. 12+10 is greater than 8, so the bov opens. It will close again once the manifold is boosted again, or the pressure equation equalizes.