The United States government has been quiet lately about the Western Sahara--since the Baker Accords, there have been no major initiatives towards independence. North-Africa.com outlines the reasons, and they're exactly what you would think--Morocco has been an ally in the War on Terror, and the United States sees no reason to stir up trouble between two of its allies, Algeria and Morocco.
The North-Africa post takes a strictly realist approach to US foreign policy: if the situation isn't damaging the United States, better not try and fix it. I think this is short-sighted, though, and you don't have to subscribe to the Wilsonian/ideological school to agree.
The United States suffers from a credibility gap, both in the MENA (Middle East-North Africa) countries and the rest of the world. We say we support democracy, while our immediate strategic objectives require supporting dictators and occupiers. Supporting Saharawi self-determination by threatening Morocco with sanctions or aid cut-offs until it holds a fair independence referendum would win admirers throughout the world, and establish an African nation that owes its very existence to the United States.
That might be too realpolitik for you. But in that case, you probably support Saharawi independence already.