I had this error in beta2 CTP June. I was trying to create an ObjectDataSource using XSD with predefined stored procedures.
One of the reason I figured is what ever the parameters you used in Insert/Update/Delete stored procedures (if you use it at all), they have to be covered in your Get stored procedure's returing columns. I.e.
Your Usp_DoSomething_Get have retured Column1, Column2, Column3
Select Column1, Column2, Column3 FROM myTable
and your Usp_DoSomething_Insert have parameters Column1, Column2, Column3, Column4
You will get this 'could not find a non-generic method' error.
My work around is you have to change your Get method to return Column4 as well.
No idea why.