If you didn't get an iPhone in the first week of it going on sale, then be prepared to wait, especially if you're in the US. According to Piper Jaffray analyst Gene Munster, it will be up to four weeks before the new device is available again.
Munster says that Apple didn't correctly predict the initial demand for the product and has been left without stock to sell. He believes four weeks is the lead time Apple needs to ramp up orders and get new units delivered to its stores. "I bet we'll see these problems for another two to four weeks. Early demand has been more than they expected. There were outages last year, but not to this extent. This is a more sustained outage, and the demand seems to be sustained. Sales internationally have taken inventory away from the U.S.," he said.
The iPhone availability link for the US shows only one in four stores have iPhones, however the UK seems to be faring better, with most stores having stock, although the 16GB black is mostly sold out.