Idea Summary
In the new APEX workflow in release 23.2 there is no feature for calling code or cancelling active tasks when a workflow is terminated using the terminate button in the Workflow Console page. 

The terminate button or APEX_WORKFLOW.TERMINATE API call simply flag the workflow status as terminated. In many cases the workflow may be related to a record and the status of that record might require “cleaning up” when a termination occurs.

Use Case
Workflow used to start different activities for a record, the initial activity changes a “status” column on the record to indicate that workflow is in process. When a user terminates the workflow the “status” column should be reset to a different value. The activity could have been a task defined in APEX and that task should also be marked as cancelled, this in turn would call the cancellation code associated with the task.

Preferred Solution (Optional)
The idea would be to add a node in the workflow builder window where processes can be called when a user terminates a workflow, ideally under the workflow tree structure, as below, perhaps named Termination Handling

The developer could then select processes similar to “Page → Processing → Process”, the process types would include Human Task - Manage, Execute Code, Invoke API, send Email, plugins etc. 

and declare the variables etc required.