NSDictionary *dict = [[NSDictionary alloc] initWithObjectsAndKeys:@"some object", @"key", nil];

if ( [dict respondsToSelector:@selector(enumerateKeysAndObjectsUsingBlock:)] ) {
    [dict enumerateKeysAndObjectsUsingBlock:^(id, id, BOOL *) {
        // dealing with the enumeration of the dict.
    }];
    [dict release];
}