Sure, you can take the commercial approach and just buy a telephoto lens for your iPhone, or hack together a macro lens, but if you're looking to combine the two into a hacked-up telephoto lens held together with a heaping of duct tape, look no further. Bhautik Joshi created the Phone-O-Scope as an experiment "to transmit light from one optical device to another to make an image". The result is an iPhone with duct-tape, cardboard, putty, PVC pipe and an SLR lens attached. It certainly won't win any design awards, but it does produce some very unique imagery.
Check out the tutorial for instructions on how to create your own, as well as side-by-side comparisons of a standard iPhone's pictures compared to the hacked-up iPhone shots.
If you're not a tinkerer at heart, you'll probably wonder why he bothered to produce an item as large as a standard DSLR but took worse photos. However, if you were one of those kids who quickly took apart your new toys just to see how they worked, this may be a little experiment you could appreciate.