Answer: First off, there's no reason to drag layers to the trash icon. You can just click the icon and it will delete the active layer (hold Option or Alt to prevent the warning dialog that usually appears). In CS, if you want to trash multiple layers, then link the layers together (by clicking the area to the right of the eyeball icon for each layer) and then hold the Command or Control key and click the trash icon to delete all the linked layers (again, hold Option/Alt to prevent the warning dialog box). In CS2 when you want to delete multiple layers, Shift-click all the layers you want to delete (which  selects them), and click the Trash icon.