SimpleFormController: onSubmit() not being called

I had a SimpleFormController and a validator. According to my logs, the command object passed validation, but I was not seeing the success-view. Upon closer inspection of the errors object in my validate method, I noticed Spring was having a problem converting the String (from the form input) to the Long property of my command class.
The solution is easy -- implement initBinder() in the controller and register a PropertyEditor.