Is there some ready-made addon that alerts admins about memcached instance being inaccessible from a Django application? I don't mean here monitoring memcached daemon itself, but something that checks if my Django app benefits from caching.
My basic idea is to check if cache.get that follow cache.set actually returns something, and if not - then send email to admins, but only one per hour, to not flood the inbox. 
But maybe there is something more advanced out there ?
You should monitor your infrastructure. You can use a huge variety of tools for this, look on server fault for more discussions on monitoring.
You should probably monitor your cache hit rate and trend it in your monitoring system; if it falls below a figure (say 90%) then you can alert that the cache has stopped working or something.
Memcached itself will have some way of monitoring hit rate, but that will be overall rather than for a specific part of your application. You probably want to monitor the hit rate for a specific cache instance in your code so you can be sure it's continuing to be effective.
Django - alert when memcached is down