[jsonDict valueForKeyPath:@"piyo.poyo"];
などで直感敵にアクセスできる
NSString *json = @"{ \"hoge\" : \"fuga\", \"piyo\" : { \"poyo\" : \"fuga\" } }";
NSError *error;
NSDictionary *jsonDict = [NSJSONSerialization JSONObjectWithData:[json dataUsingEncoding:NSUTF8StringEncoding] options:0 error:&error];
NSString *value = [jsonDict valueForKeyPath:@"piyo.poyo"];
NSLog(@"val: %@", value); // val: fuga