By initiative, California voters legalized the use of marijuana under a doctor's supervision. California voters cannot change federal law and today, the US Supreme Court held that federal law preempts state law and users of medical marijuana can still be prosecuted under federal statutes.
The majority opinion was written by John Paul Stevens followed well-established law on the preemption clause. The federal law preempts state law where the federal government has shown an intent to occupy that area of law. The federal drug laws do that and Stevens held that the proper remedy is for Congress to change the law, not to favor state laws in a preempted area.
Sanda Day O'Connor wrote the dissent. Apparently not agreeing that the federal government had preempted the drug arena, she argued for states' rights in setting their own criminal laws.
UPDATE: THe Modesto Bee has excerpts from the opinion. Justice Thomas also dissented.