So why is Apple CEO Steve Jobs saying sorry?
The device, which features video-calling functionality and multitasking among other enhancements over previous models, caused long queues of fans outside some Apple stores on the day it went on sale.
Apple CEO Steve Jobs described it as the most successful product launch in Apple's history but added in a statement: "Even so, we apologise to those customers who were turned away because we did not have enough supply."
But it hasn't all been plain sailing for the new handset: users have reported problems with their iPhone 4's signal reception when holding the device in a particular way.
The phone features a flat, stainless steel band running all around its edge. The metal strip is used to make the handset more robust but also acts as part of the iPhone's antenna system, according to Apple.
When a user covers the lower left portion of the metal band while simultaneously touching a portion of the strip on the right-hand side, it can cause issues with the iPhone's signal, according to the company, which insisted that reception problems are "a fact of life" for all mobile phones.
It was also recently revealed that government ministers are not allowed to use iPhones because they have not been cleared for use by CESG, the information assurance arm of British intelligence agency GCHQ.