Tehnically, US Supreme Court would be ruling merely on whether California's Proposition 8 (marriage is only recognized between men and women) is constitutional.
Many other states have laws etc... which deal with same sex marriage. The full breakdown is here:
Is there any legal analysis which indicates how a possible "nay" ruling from SCOTUS would affect specific laws in the other states?
(intuitively, it would seem all of them would be voided, but the devil in law is frequently in silly technicalities).