Apple has shifted over one million iPhone 3GS models in just three days of trading. The company has also announced that over six million customers have downloaded the new iPhone 3.0 software in the five days since its release.

The Cupertino, California-based firm revamped its smartphone device, introducing a new model that is said to be twice as fast as its predecessor with a longer battery life, a 3 megapixel camera capable of recording video and hands-free voice control.

Apple’s new mobile OS, version 3.0, features a number of new features, such as copy, cut and paste, which brings the iPhone in line with other smartphones, and MMS.

Users will have access to Apple’s App Store, which recently celebrated its billionth download and now features over 50,000 apps to choose from.

“Customers are voting and the iPhone is winning,” said Steve Jobs, Apple’s CEO. “With over 50,000 applications available from Apple’s revolutionary App Store, iPhone momentum is stronger than ever.”

The news is a timely boost for Apple after more reports emerged at the weekend concerning the health of founder and CEO Steve Jobs. The Wall Street Journal revealed on Saturday that Jobs underwent a liver transplant about two months ago. Apple refused to comment on the stories, insisting that: “Steve continues to look forward to returning to Apple at the end of June. There is nothing further to say.”

Jobs survived pancreatic cancer in 2004 but suffered dramatic weight loss over the last 18 months, prompting more fears for his health. In January 2009 he announced he was suffering from a hormone imbalance and shortly after this it was revealed Jobs would be taking a medical leave of absence from Apple until the end of June.