No. ArcGISDynamic and ArcGISTiled services, you cannot change labelling on the fly. Labelling support is on our roadmap for FeatureLayer the final release of v2.0.
Until then, you could use a graphics layer with points that uses a text symbol which binds to an attribute. However if your data is not points, it's currently on you to find the label placement within the polygon or polyline.
