Ah. So can state constitution overpower federal at all? I.e. if Capitol hill passed a law allowing gay marriage (yes, yes, unlikely as it is), would NC be allowed to refuse? I'm assuming not, but your country's politics are pretty arcane...
Therein lies the rub. Federal law trumps state constitution, but the way it's written, all powers not explicitly granted to the US Congress are supposed to devolve to the states. This is the crux of the whole "States' Rights" argument. States (particularly the gay-hatin' ones) are going to argue that since there is nothing specifically in the Constitution empowering Congress to determine the definition of "marriage", then it is left to each state to decide. Congress, for its part, could probably argue that it falls either under interstate commerce (because we have a patchwork where a couple could be legally married in their home state and not in another state) or that such state constitutions are in violation of the Constitution, specifically with regards to equal protections and "pursuit of happiness".
Last time we had a situation where someone could have different legal statuses in different states, it was over slavery. We know how that turned out. There's already been an instance I know of where a lesbian couple were on vacation in a state that didn't recognize it (I want to say either Texas or Florida) and one fell gravely ill. The hospital and local law enforcement refused to allow her partner any kind of visitation rights, and she finally died in the hospital, alone. IIRC, the lawsuit failed in state court, because they were in accordance with state law. Dunno what ever became of that case, but I'm guessing it didn't go to Federal court.
@GlyphGryph: Actually, states are not legally bound to honor marriages from other states or countries. They do so as customary law, but AFAIK this amendment specifically bars North Carolina from legally recognizing same-sex marriages from other states.
EDIT: Also, a quick note to say that although the Dems may stand to pick up a seat now that Dick Lugar is out, the man deserves some credit: he was instrumental in crafting the legislation that allowed us to help the remnants of the Soviet Union safely downsize their nuclear arsenal. That's no small thing.