Since you're already using AWS, this sounds like a perfect use case for Amazon Simple Workflow Service (http://aws.amazon.com/swf/)
From the service description:
Amazon SWF replaces the complexity of custom-coded workflow solutions and process automation software with a fully managed web service. This eliminates the need for developers to manage the infrastructure plumbing of process automation so they can focus their energy on the unique functionality of their application.
It allows you to define task workflows and have workers process tasks in the workflow.
boto
has 'level1' support (low level api) for SWF: http://boto.readthedocs.org/en/2.6.0/ref/swf.html