In addition to the speculation about what happens to existing same-sex marriages in California (the options are: turned into the old DP's, voided, left standing but not 'recognized' in California, or, unlikely but suggested by some, still recognized but no new marriages issued), there is also the question of what prop 8 does to DP's if they want to go back to them. Does Prop 8 make DP's unconstitutional also? Well, no, it doesn't, but even so, they can't go back to those old DP's as they'd been defined either, because those were already unconstitutional.
The court said it is unconstitutional discrimination to call identical legal institutions different names for same-sex couples. Proposition 8 didn't amend the constitution to say that it was OK to now call identical institutions by different names, it said that only a man and a woman can be recognized as legally married. So it would still be just as unconstitutional now as it was seven months ago to use a different name for the same rights. If that opinion was right then (and I think it was) then it is still right today.
So uh-oh, it looks like there is a bit of a bind here. The only constitutional option is to make them unequal institutions, with different legal rights and protections. The best way to do that is with a "marriage minus one" formulation, so that all the other rights are known to be included. The right has to be intrinsic to marriage in order for it to be subtractable from marriage, such as the right not to self-incriminate your spouse, or the right to inherit property. It wouldn't work to say "marriage minus the right to vote", because that is an individual right, something done by individuals. It has to be something marriages have a right to do together, and something that same-sex couples don't want or need, or it would defeat the purpose.
How about...the right to conceive a child together using the couple's own genes? This has the advantage over other possibilities because it's something we need to ban as soon as possible anyhow, and there would be lots of long lasting benefits to banning it.
California should be the first state to enact "Federal Recognition Ready" Civil Unions that are defined as marriage minus the right to conceive children together, so that when Congress enacts the Egg and Sperm Law and repeals DOMA, their Civil Unions will match the necessary definition to be recognized as marriage and they could qualify immediately. Then other states would follow suit soon after.