Jeremy Dunck wrote this cache backend for Pegasus News, and I've been given permission to open-source it. I'm going to create a full Github repository with a setup.py for easy pip install, and I also plan to contribute it to Django. For now though, I'm

prefixed.py

import types
from django.conf import settings
from urllib import urlencode
from django.core.cache.backends.base import BaseCache
from django.utils.encoding import smart_str

#future-proofing against any Django upstream additions of new cache methods.
# If we didn't do this, we'd silently use the base class's methods, which wouldn't
#  use the prefix as required.
_expected_methods = set(['__contains__', '__init__', 'add', 'close',
    'decr', 'delete', 'get', 'get_many', 'has_key', 'incr', 'set',
    '_get_memcache_timeout', 'clear', 'delete_many', 'set_many'])

class CacheClass(BaseCache):
    def __init__(self, server, params):
        #e.g. CACHE_BACKEND="path.to.prefixed://memcached://10.0.0.3:11211/?prefix=xxx"
        #  Falls back to settings.SITE_ID if no specific prefix is given.
        from django.core.cache import get_cache
        self.prefix = smart_str(params.pop('prefix', str(settings.SITE_ID) + ':'))
        backend_uri = server
        if params:
            backend_uri += "?%s" % urlencode(params)
        self.backend = get_cache(backend_uri)
        actual_methods = set([m for m in dir(self.backend)
                            if isinstance(getattr(self.backend, m), types.MethodType)])
        new_methods = actual_methods - _expected_methods
        assert not bool(new_methods), "New methods have been added to cache backend: %s." % new_methods
        super(CacheClass, self).__init__(params)

    def get(self, key, default=None):
        return self.backend.get(self.prefix + key, default)

    def set(self, key, value, timeout=0):
        return self.backend.set(self.prefix + key, value,  timeout)

    def delete(self, key):
        self.backend.delete(self.prefix + key)

    def add(self, key, value, timeout=None):
        self.backend.add(self.prefix + key, value, timeout)

    def get_many(self, keys):
        l_prefix = len(self.prefix)
        _keys = [(self.prefix + key) for key in keys]
        _results = self.backend.get_many(_keys)
        for k in _keys:
            if k in _results:
                _results[k[l_prefix:]] = _results[k]
                del _results[k]
        return _results

    def has_key(self, key):
        return self.backend.has_key(self.prefix + key)

    def incr(self, key, delta=1):
        return self.backend.incr(self.prefix + key, delta)

    def decr(self, key, delta=1):
        return self.backend.incr(self.prefix + key, -delta)

    def __contains__(self, key):
        return self.backend.has_key(self.prefix + key)
    
    def delete_many(self, keys):
        _keys = [(self.prefix + key) for key in keys] 
        self._cache.delete_multi(map(smart_str, _keys))
    
    def set_many(self, data, timeout=0):
        safe_data = {}
        for key, value in data.items():
            if isinstance(value, unicode):
                value = value.encode('utf-8')
            safe_data[smart_str(self.prefix + key)] = value
        self._cache.set_multi(safe_data, self._get_memcache_timeout(timeout))