Few comments:
What if someone wrote a trojan/virus that only attacked jailbroken phones?
The way that apps are distributed from a central repository, or even on jailbroken phones from web sites, makes it hard to see how malware could propagate from one phone to the next.
A trojan that gathered valuable info from a phone and sent it to a server might be a more likely strategy.
There is code that runs in the sandbox that can access data and hardware from outside the sandbox. This includes everything from the size and orientation of the display, taking a picture, getting files from the camera roll, and a bunch more. There could be security holes in any of the UIKit code that does those things. So it might be possible to find out how some of that functionality works and duplicate it or subvert it. FWIW, I've never tried to do anything like that and Apple's engineers aren't dummies to it's certainly going to be hard and might be impossible. gdb can connect to your device over USB and do more-or-less what it likes.
Given Apple's control over the distribution of software it seems like it would be impossible to distribute malware and completely get away with it. They would figure out how it was done and who did it. OTOH, it should be impossible to jailbreak phones and it would seem that Apple would have a great motivation to fix that problem but they appear to do nothing about it.
There's no malware on MacOS X so it must be a hard task.