Django-resolvetime is a Django filter to convert a python datetime object into a format that specifies the time difference between the time of the datetime and now. I developed this for a personal project of mine and decided to opensource since I could not find a filter available to get the job done.
The code is hosted at http://code.google.com/p/django-resolvetime/. Installation and usage of the filter is straight forward. Instructions are given in the README file.
Kasun Herath 
I was recently looking for this exact filtering functionality. Thanks!
http://docs.djangoproject.com/en/1.1/ref/templates/builtins/#timesince
@John
Thanks for pointing out. I had missed the built in filter!
The timesince filter is minute precision meaning the smallest unit is minutes. I see your code supports seconds. For situations where time difference of seconds is required I will be using your code.