As opponents of Proposition 8 fight a last-ditch battle to overturn the results (on the grounds that the amendment is actually a "revision" to the California constitution, which would require a 2/3 majority to pass), proponents are trying to press their own advantage further: seeking to nullify the gay and lesbian marriages that already occurred under California law. Their argument is that the text of the amendment prohibits recognition of all marriages that aren't between one man and one woman, and that this necessarily applies retroactively.
Let's put aside a moment the fact that these people are assholes (I know it's hard, but let's try). My understanding (probably incorrect) was that the state is constitutionally precluded from nullifying previously created contracts. See, e.g., Dartmouth College v. Woodward, 17 U.S 518 (1819); Fletcher v. Peck, 10 U.S. 87 (1810). In Fletcher, that included a repealing a corrupt state grant of land, so the prohibition is pretty broad in scope. The marriage contracts entered into between the court's decision and Proposition 8 were valid under California law at the time, hence, I have difficulty understanding how the state could retroactively annul them without running afoul of some very old contracts clause precedents.