Removing the massive amount of for-loops in this code
There are a few tweaks that can improve this code:

def add_random_fields():
    from numpy.random import rand
    server = couchdb.Server()
    databases = (server[database] for database in server if not database.startswith('_'))
    # immediately fetch the Server object rather then the key, 
    # changes to a generator to avoid instantiating all the database objects at once
    for database in databases:
        for document in couchdb_pager(database):
            document = database[document] 
            # same here, avoid holding keys when you can have the objects

            for tweet in document.get('results', []):
            # rather then checking for the key, have it return a default that does the same thing
                if tweet and 'rand_num' not in tweet:
                    print document
                    tweet['rand_num'] = rand()
                    database.save(tweet)