A video showing the crack in action has been posted on Vimeo.
The cracking technique reportedly involves hitting the 'emergency call' soft key and then rapidly hitting `###', followed by 'call' – then briefly depressing the lock key on the top of the smartphone.
From there, the user's memory directory opens and anyone can then make a call from the directory, or place a call using the iPhone keypad as normal.
Infosecurity can confirm the flaw appears to be limited to any iPhone (from the 3 model upwards) running iOS4, the latest iPhone operating system that appeared when the iPhone 4 was launched this summer.
It also seems that no other applications can be run on the iPhone using this technique, meaning that email and text messages appear to be secure, although the ability run up a large bill on an iPhone users' tab is a potentially major issue.
Various discussion forums suggest that Apple has known about the issue for a short while, and fixed the bug in the beta 3 edition of iOS 4.2, which is circulating amongst developers at the moment.
Apple has yet to make a statement on the problem.