Hmm. OK. Try this... close all instances of Visual Studio, and if you are running your service in IIS, stop that website in IIS Manager... then browse to "C:\temp" and delete all the .dll files (make sure all them go away). If it doesn't let you delete them all (due to some process hanging on to them), you might have to log off and back on to delete them all.
Once those are all deleted, start your web service back up again, and then try again with WW.Interface...
It seems like it's possible that you had, at one time, generated this back end with a previous version of WW.DataServices, and that dynamic assembly got cached...
Let me know if that helps.
-Ryan