Well, San Francisco "City Attorney Dennis Herrera will file [a] legal challenge in the California Supreme Court if the measure passes." The argument, as I understand it, is that Prop. 8 is not merely an amendment to the state constitution, but a revision thereof, and only simple amendments can be done by ballot initiative -- at least, that was the basis of a legal challenge when the proposition was put on the ballot, which the Court decided not to hear apparently on grounds of not wanting to deal with it until it was voted on. There may also be an argument against the proposition on grounds that it leaves things not internally consistent.
Having read most of the information I can find on the amendment-vs.-revision distinction, I don't think it's at all clear which way the Court is likely to decide on that issue. Or, for that matter, if it is declared an amendment, whether or not taking it back off the constitution would also be an amendment.
Meanwhile: Seventy million dollars? Spent on this? That amounts to about seven dollars for every voter who voted on the issue. I find this absurd, and a bit sad.