There are lots of possible reasons for an imbalance between iPhone demand and supply:
...AT&T retail stores in Manhattan aren't yet feeling the affects of the shortage, though Apple's online store is also reflecting an approximate 1 week delay for all new orders, suggesting that considerable backlog currently exists for whatever reason...
One of the possible explanations is that Apple is about to launch a new model.
I have been expecting a new model to be announced on June 31st and to be generally available around August, but I'm a pessimist. If you're considering buying an iPhone I'd wait a couple of weeks. If new models are coming out then the better Apple rumor sites expect the 16GB model to continue to be sold at a lower price point, and the 8GB model to be discontinued.