Create a basemap that also includes a reference layer (e.g., Esri's "Imagery + Labels" basemap). Currently (June 2013), this can only be done programmatically.

gistfile1.py

   def addItem(self):
        ''' Uses the addItem to create new content in an Org. '''
       # http://www.arcgis.com/apidocs/rest/index.html?additem.html
       # replace - http://tiles.arcgis.com/tiles/.../MapServer
       # with the tile layer you want to use.  Feature services will not work.    
 
        portalId = self.__portalId__()
 
        req = urllib2.Request( self.portalUrl + '/sharing/rest/content/users/'+self.username+'/addItem')
        req.add_data(urllib.urlencode({'token' : self.token,
                                       'f' : 'json',
                                       'text' : '{\"operationalLayers\": [],\"baseMap\": {\"title\": \"Light Gray Canvas Custom\",\"baseMapLayers\": [{\"visibility\": true,\"opacity\": 1,\"url\": \"http://services.arcgisonline.com/ArcGIS/rest/services/Canvas/World_Light_Gray_Base/MapServer\"},{\"visibility\": true,\"opacity\": 1,\"isReference\": true,\"url\": \"http://tiles.arcgis.com/tiles/.../MapServer\"}]},\"version\": \"1.4\"}',
                                       'title' : 'CustomBasemap2',
                                       'type' : 'Web Map',
                                       'tags' : 'basemap'}))
 
        print urllib2.urlopen(req)          
       
        return
 
# sample call to addItem
import csv
import admin
myAgol = admin.admin()
myAgol.addItem()