Todo app for displaying a list of tasks
I would suggest moving the CSS selection logic to a custom template tag or even a filter, e.g. &lt;span class="{{ t|due_date_color }}"&gt;{{ t.relative_due_date }}&lt;/span&gt;. After that your templates will look much cleaner and that logic would still belong to the presentation part of MVT (Model, View, Template).
You can split your tuple in the definition of the loop so that you don't redefine the tasks variable. {% for header, task_list in tasks %} .... 
Use reverse URLs, no question about it. You'd hate the day you have to change them otherwise :) Go even further, use URL namespaces.