A full OS X with the iPhone GUI would be fantastic, and relatively easily accomplishable.
You've never designed any software have you?
Although you don’t see it with your eyes, the sizes of the keys on the iPhone keyboard are changing all the time. That is, the software enlarges the “landing area” of certain keys, based on probability. For example, supposed you type “tim.” Now, the iPhone knows that no word in the language begins timw or timr—and so, invisibly, it enlarges the “landing area” of the E key, which greatly diminishes your chances of making a typo on that last letter.
I personally haven't heard of any exploit in the wild except the trojan, for which the user has to be willing to provide their password to any old bit of software with unknown providence - to be honest I don't know how one could protect against that on any system.
Luckily, Ivan Krstic knows how. From a CNET article about Bitfrost:
Instead of blocking specific viruses, the system (Bitfrost) sequesters every program on the computer in a separate virtual operating system, preventing any program from damaging the computer, stealing files, or spying on the user. Viruses are left isolated and impotent, unable to execute their code.
The most difficult thing in the world is to know how to do a thing and to watch someone else doing it wrong, without commenting. -- T.H. White