I don't recall which decisions exactly, but sometime in the last year or so at least one of the justices on the USSC (Stevens?) have referred to some international law stuff... I think it may have been with regard to the death penalty somehow.
Stevens relied upon international standards (i.e. what the Euros think) to support his opinion that executing a 17 year old was 'cruel and unusual' punishment. He's an idiot, but that is still not "relying on UN laws".