How difficult is it to jailbreak an iPhone, install a trojan, and then make it superficially at least unjailbroken?
At the least steps 1 & 2 ought to be pretty easy.
At the least steps 1 & 2 ought to be pretty easy.
Then good luck hiding your trail, reassociating the Apple ID, hoping no apps broke due to the jailbreak (it's happened to me), keeping the device associated with the owner's iTunes, etc.
I'm not sure if there are any trojans available, but if there were, you could easily install them and then either delete the jailbroken app store or just hide it from SpringBoard.
This process could easily be done in 10-15 minutes as long as you have physical access to the phone.