Omigodomigodomigod - The guts of a fully-functional layer switcher build onto mapbox.js - via the good folks at MotherJones

switch.js

//Establish URLs and names for all layers
  var layer_names = {
    //Base layer
		'tasneemraja.map-7lrzv81u':'base',
    
    //Overlays
		'tasneemraja.election-2012-voter-regi':'Tougher Registration Laws',
		'tasneemraja.election-2012-voter-id':'Voter ID Required',
		'tasneemraja.felons':'Disenfranchised Felons',
		'tasneemraja.election-2012-early-absentee':'Early and Absentee Restrictions',
		'tasneemraja.counties-election-results-2008':'2008 Election Results'
		}

//	var marker_colors = {
//			'white':'#fff',
//			'black':'#000',
//			'blue':'#73b5e5',
			}

  for (var i = 0; i < possible_layers.length; i++) {
      var n = possible_layers[i];
      var item = document.createElement('li');
      var layer = document.createElement('a');
          layer.href = '#';
          layer.id = n; 
          layer.className = ''; 
          layer.innerHTML = layer_names[n];

      layer.onclick = function(e) {
          e.preventDefault();
          e.stopPropagation();
          var listitems = document.getElementsByClassName('active');
          for (var i = 0; i < listitems.length; i++) {
              listitems[i].className = '';
          };

          mapbox.load(this.id, function(o) {
              var layers = map.getLayers();
              for (var i = 0; i < layers.length; i++) {
                  for (var j = 0; j < possible_layers.length; j++) {
                      if (layers[i].name === possible_layers[j]) {
                          map.removeLayer(layers[i].name);
                      }
                  }
              }	
              removeLegend();
              map.addTileLayer(o.layer);
              addLegend(o.legend);

              map.interaction.auto(); 
          });
          this.className = 'active';
      };

      item.appendChild(layer);
      layers.appendChild(item);
  }