Technically, lesbian and gay Californians who get a domestic partnership get all the rights the state provides married straight couples.
(How you get it is rather different, of course: you download your DP form from the internet, rather than go personally to the county clerk; notarize it at Kinko's, rather than have a JP, marriage commissioner, or minister perform a marriage, and then you mail in to the state with a check, rather than file with the county to be sent a proper copy. But i digress.)
So, there is really nothing that marriage in the state of California confers that is different than a DP.
And that may be the crux of the appeal. Because with Prop8, the voters did not take away any of those RIGHTS associated with marriage--the California Supreme Court ensured that. They simply took away the WORD marriage, which is freighted with symbolic meaning. As Attorney Therese Stewart argued for the plaintiffs, the only reason to do that was to deny lesbian and gay Californians access to the symbolism: they wanted them to be treated as a separate, lesser class.
The questioning by the judges in the district court suggest that this affirmative act of taking away the symbolism of the word, and not anything of substance, may make Prop8 particularly vulnerable on Constitutional grounds.
Taken further, this viewpoint would suggest that once you give LGBT people any partnership rights, then you are creating them as a second class by denying them marriage. By this argument, states with DPs may indeed be on the "slippery slope" to marriage equality, while those states that deny their LGBT citizens any protections may be inoculated.
This does not get into the thorny area of "strict scrutiny" of LGBT people as a class or equality issues overall. While that's the question Ted Olson wants to litigate (one that could have nation-wide significance to the equality movement), that may not be the question upon which these judges will rule.
Instead, it may all hinge on the fact that we had the right to get married--and 18,000 of us did. And then the voters took it away.