Answer
Yes, layers can be dragged and dropped from one data frame to another, from one map to another, and from the Contents pane in ArcCatalog to ArcMap.
By default, only one layer can be dragged and dropped from the ArcMap 10 Catalog window into the current ArcMap map document. To drag and drop multiple layers into the current ArcMap map document, change the Catalog window view so that the Contents pane is visible.