Workflows demand that your task list be based directly off of the Tasks content type.  To trigger this message, I created a content type called “Review Tasks” that inherited from the “Tasks” content type and then based my task list on “Review Tasks”:
The BaseTemplate property of the list is not 'Tasks'

Instead of recreating my list definition, I tried simply modified the schema.xml to point directly at the Tasks content type, like so, no dice:

So I stepped through the List Instance creation wizard again and this time, ignored by list definition and instead just picked “Tasks”:

Perfect.  Lesson, even if your only reason for a intermediary content type is to try and follow the best practices set out by Microsoft.. When it comes to Task lists that will be leveraged by your out of the box SharePoint workflows, stick to basing the list instance directly on the “Tasks” list definition available out of the box…