There's an interesting article about Opera at the New York Times BITS blog from yesterday. Of particular interest to those who have been asking for Opera Mini for the iPhone is this part:
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's pretty well known by now that Apple blocks competitors from their store. I'm not sure if we've ever confirmed that we actually had Opera Mini ready for the iPhone, but now you know that it did exist.
And now you know what keeps you from running Opera on your iPhone.
Update: It seems that the New York Times misquoted Jon. However, Apple's terms still block competitors from their App Store.