You can use them on iOS 5.1 by implementing the methods the subscripts map to on NSArray and NSDictionary (as well as mutable) in categories.

For NSArray:

    - (id)objectAtIndexedSubscript:(NSUInteger)index {
        return [self objectAtIndex:index];
    }

NSMutableArray:

    - (void)setObject:(id)object atIndexedSubscript:(NSUInteger)index {
        if (index < self.count){
            if (object)
                [self replaceObjectAtIndex:index withObject:object];
            else
                [self removeObjectAtIndex:index];
        } else {
            [self addObject:object];
        }
    }

NSDictionary:

    - (id)objectForKeyedSubscript:(id)key {
        return [self objectForKey:key];
    }

NSMutableDictionary:

    - (void)setObject:(id)object forKeyedSubscript:(id)key {
        [self setObject:object forKey:key];
    }

The functionality is present for iOS, it just isn't being exposed. If you create a category header file (just header, not implementation) the compiler'll pick up on it and all will work fine.

This isn't my code, but I've been happily using it for a week or so now: https://gist.github.com/3299750

Apparently OSX 10.8 and iOS 6 bring support for subscripting to native NSArray/NSDictionary, but I think you can add subscripts to your own classes already just by implementing the following methods:

  -(id)objectAtIndexedSubscript:(NSUInteger)index;
  -(void)setObject:(id)object atIndexedSubscript:(NSUInteger)index;

as weiran has shown for NS(Mutable)Array/NS(Mutable)Dictionary.

That functionality is coming with Xcode 4.5