Use banshee version banshee 2.0.0-1ubuntu2 on Ubuntu 11.04 I was trying to set the rating via the command line and received the following error.

8:47:43 - flash:[~] banshee --debug --set-rating=2
** Running Mono with --debug **
[1 Debug 08:47:48.766] Bus.Session.RequestName ('org.bansheeproject.Banshee') replied with InQueue

Unhandled Exception: System.ArgumentNullException: Cannot handle a null message; maybe the bus was disconnected
Parameter name: msg
  at NDesk.DBus.Connection.HandleMessage (NDesk.DBus.Message msg) [0x00000] in <filename unknown>:0
  at NDesk.DBus.PendingCall.get_Reply () [0x00000] in <filename unknown>:0
  at NDesk.DBus.Connection.SendWithReplyAndBlock (NDesk.DBus.Message msg) [0x00000] in <filename unknown>:0
  at NDesk.DBus.BusObject.SendMethodCall (System.String iface, System.String member, System.String inSigStr, NDesk.DBus.MessageWriter writer, System.Type retType, System.Exception& exception) [0x00000] in <filename unknown>:0
  at Banshee.MediaEngine.IPlayerEngineServiceProxy.set_Rating (Byte ) [0x00000] in <filename unknown>:0
  at Halie.Client.HandlePlayerCommands () [0x00000] in <filename unknown>:0
  at Halie.Client.Main () [0x00000] in <filename unknown>:0
  at (wrapper managed-to-native) System.AppDomain:ExecuteAssembly (System.Reflection.Assembly,string[])
  at System.AppDomain.ExecuteAssemblyInternal (System.Reflection.Assembly a, System.String[] args) [0x00000] in <filename unknown>:0
  at System.AppDomain.ExecuteAssembly (System.String assemblyFile, System.Security.Policy.Evidence assemblySecurity, System.String[] args) [0x00000] in <filename unknown>:0
  at (wrapper remoting-invoke-with-check) System.AppDomain:ExecuteAssembly (string,System.Security.Policy.Evidence,string[])
  at System.AppDomain.ExecuteAssembly (System.String assemblyFile) [0x00000] in <filename unknown>:0
  at (wrapper remoting-invoke-with-check) System.AppDomain:ExecuteAssembly (string)
  at Booter.Booter.BootClient (System.String clientName) [0x00000] in <filename unknown>:0
  at Booter.Booter.Main () [0x00000] in <filename unknown>:0

I've also reported this bug in Launchpad at https://bugs.launchpad.net/ubuntu/+source/banshee/+bug/761945.