In an article from the New York Times on the Opera Software company it is revealed that Apple is not allowing the company to distribute a version of its web browser for the iPhone.
Mr. von Tetzchner said that Opera’s engineers have developed a version of Opera Mini that can run on an Apple iPhone, but Apple won’t let the company release it because it competes with Apple’s own Safari browser.
It is speculated that the decision has been made by Apple because the company doesn't want applications available that compete with the pre-loaded apps.