by SCOTUS on a national level b/c there's a circuit split in the federal courts that have dealt w/challenges to the various states' refusal to issue the licenses. those states are sued in federal court which makes the issue federal. when appealed those decisions go to federal appellate courts. the various courts have split on how they decide the issue.

also there is federal implication via the Full Faith and Credit Clause, i believe (http://en.wikipedia.org/wiki/Full_Faith_and_Credit_Clause). generally the various states recognize each others' marriage licenses but when it comes to same sex licenses some states have refused to recognize those of other states. that causes a federal problem (though the issue is unclear as of yet).

but anyway, yeah...it's a state's rights issue that has federal implications.