Well, there have been vulnerabilities for iPhone that allowed remote exploits. One could be triggered by text message. When it was demonstrated that the microphone could be remotely activated and start transmitting, it even inspired some parts of The Dark Knight.
However, iOS was increasingly locked down after the first iPhone. If such a vulnerability still exists in iOS and is discovered, it would probably be immediately abused. We'd probably see an extremely devastating virus or wide-spread botnet almost overnight.