I just released an implementation of DurationField for Django to PyPi.
The latest release uses datetime.timedelta for its internal representation of durations (thanks to Paul Oswald) and has support for South thanks to Wes Winham.
The documentation is now hosted on ReadTheDocs and the package data is available on Django Packages.
After a discussion with a few Django core developers, it seems like keeping the DurationField implementation as a separate, reusable application is the preferred option. By staying independent, we keep the ability to make changes independent of the Django release cycle. Additionally, we avoid adding bloat to Django-core. Since not all databases implement an interval or duration data type (PostgreSQL does), django-durationfield is in some ways a hack by using a bigint datatype to store integer instances of timedelta.