I'm no legal scholar but doesn't the US Constitution prohibit ex post facto laws? If that's true then there's no was you can invalidate,within the state of California,"marriages" that occurred between the time when they were declared legal by the CA Supreme Court and the day that Prop 8 passed.
This isn't a matter of ex post facto application. The state will not be criminalizing a same-sex marriage that occurred before the amendment passed. It only says that such a marriage will not be recognized by the state.
It works the same as passage of Amendment XIII abolishing slavery. Just because someone happened to be a slave at the time that amendment was ratified does not mean that person remained a slave.