Thanks to a little quirk within the memcache backend of Django caching with the timeout 0 actually caches with the default-timeout. Since this behaviour seems to be limited to the memcache backend, a cache_time of 0 should circumvent the whole caching infrastructure while None should use the default timeout.
Thanks to Benjie Chen for reporting this.
If cache_time is set to 0, don't even try caching. Closing #7