Jason,Here's a link to a sample that shows how to add and remove features from the current selection. In this example click on the map to select features then shift-click to remove features from the selection. http://jsfiddle.net/J4sfK/
dojo.connect(neighborhoods,'onClick',function(e){
var query = new esri.tasks.Query();
query.objectIds = [e.graphic.attributes.FID];
//if shift key is pressed subtract from selection - otherwise add
if(e.shiftKey){
neighborhoods.selectFeatures(query, esri.layers.FeatureLayer.SELECTION_SUBTRACT);
}else{
neighborhoods.selectFeatures(query, esri.layers.FeatureLayer.SELECTION_ADD);
}
});