It depends - if you are already authenticated as a different user when
you login, then your session is flushed, but you keep the same session
key. If you are not already authenticated, then you keep your session,
but your session key is changed.
Neither of these details are mentioned in the docs:
https://docs.djangoproject.com/en/1.3/topics/auth/#django.contrib.auth.login
Cheers
Tom