And program it to drive to their house while I sleep?
No.
Really? The other day I watch ed an old Mythbusters on Netflix, where they made quite the rig to turn an old Plymouth Fury into a remote control car. No reason that wouldn’t work with a Prius, all you need is access to the car to set it up, and a way to start the car. The iphone could certainly be used as the remote control device.
That is an assumption that’s absolutely not made by the OP.
I could turn your house into a completely locked down Panic Room-style building where you’d be trapped for the next 200 years by 2 inch think steel walls and control it with my iPhone, if I had 24 hours of unmonitored access to your house, the resources necessary and the required manpower - then hook it up to an iPhone controlled application. But that doesn’t mean I “could hack your house with an iPhone” without a giant list of concessions.
This is like saying “Oh, that virus software would never work. After all, they could just put the virus on a disk, walk up to your computer, pop in the disk and load it straight into the boot sector. All they would need is physical access to the computer.” Clearly, physical access lets you do just about anything.
Regarding the OP, the answer is almost certainly no. Unless you have some reason to believe otherwise?
Maybe you have some special definition of hack other than the generally accepted one? I don’t recall any limitations specified in the OP.
Of course it is. All it said was his iphone, and his Prius (or hers, I dunno). It didn’t rule out physical access to either, or even social engineering to accomplish the task. Maybe I can get OP’s wife to respond to a call on his phone while he sleeps and drive the car over to me. I am pretty sure social engineering falls under the generally accepted rubric of hacking. At least it does in all the hacking magazines and web sites I have ever read 
Done. You should be all set. That’ll be one billllion dollars, please.
The limitation is right there in the OP: “Could someone hack my Prius over my iPhone?”
That implies the hacking itself is done using the iPhone. The question being asked appears to be: given unlimited access to my iPhone, can you hack into my Prius and program it to drive it to your house?
I suspect the question comes from trying to understand the nature of a Prius versus a normal car in terms of the scope of electronic controls and a hackable software interface to these controls. If the question includes building a giant rig, something you could do for any car, it becomes significantly less interesting and it is unclear why the Prius was singled out. It seems clear that this wasn’t the intent, and “no” is the correct answer: there is no hackable interface in a Prius that will allow you throttle and steering control to the extent that you could remotely drive the car anywhere.
You don’t think the OP meant “park the Prius over the iPhone, then do all the stuff not_alice was talking about”?
In terms of physical connectivity to control systems, it is closer than you might think. Rather than a Prius, if I think about my Lexus is250 (also a Toyota) I can just about put together a physical set of links that could get from an iPhone to an ability to control almost all of the car’s functions.
The audio system is an integrated set of bits that also provides bluetooth connectivity to the phone for the handsfree phone function, the same display controller also runs the climate control user interface. Now access to that will almost certainly be over CAN bus. Just how many CAN busses there are in the car, and whether there are any accessible devices that sit astride them, I don’t know. But pretty well any modern car is controlled using CAN bus, or possibly FLEXRay for high bandwidth needs. This includes just about every switch on the dashboard, and most of the functions of the car, from simple things like the door locks, right through to anti-skid control and engine management. Cars with electrical power assisted steering potentially have control over steering functions - witness the Volkswagen plethora of cars with auto park capability. The anti-skid has access to brakes, and my car (and a Prius) has an electronic throttle. Plus it also has keyless start.
So, if we imagine a series of software hacks that take control and reprogram each of the control computers in the car, is there a theoretical path, one that requires no changes to hardware, that could gain control over a car’s functions? The answer is maybe yes.
You can buy CAN bus WiFi access points. These may be used to provide after market access to the engine management sensors and operating parameters - and there are after market displays that allow you to display these on the daskboard. There are, surprise surprise, iPhone apps for this purpose. I was amused and dismayed to discover that most of these access points simply create an open WiFi access point, with no attempt at security.
Of course, the idea that there are open holes in the software of the various controllers, starting at the Bluetooth interface and running all the way through the car that would allow me to inject arbitrary control packets is stretching things.
Even if we conceded for the sake of argument that you could somehow hack into it and obtain remote control over the steering, throttle, etc. you wouldn’t get far considering that you’d be driving blind.
Does the Prius have or allow electronic control of its steering?
That seems easy to deal with: you show up in the street outside Cubsfans’s house, back the Prius out of the driveway, and follow it home (providing continuous control inputs). It isn’t quite what was suggested in the OP, but still a rather nefarious scheme.
Some models have automatic parallel parking, so I think the answer to this specific point is (a perhaps limited) yes.
If you’re going to do that then why not just hack the door locks and starter and get in and drive it home.
Oh come on - that’s not nearly as impressive. Nor would it allow you to guide a convoy of a half-dozen hijacked driverless Priuses.
I see other uses for this iPhone app:
Slow Prius is hogging the passing lane, and you can’t get by. You wait for a safe opening, then cause the Prius to drift gently aside, leaving your lane clear.
You’re looking for a parking spot - and find one occupied by a Prius. You cause the Prius to move into an illegal slot, then grab the legal one.
Cop has nabbed you for running a stop sign and is preparing to write an expensive ticket. All of a sudden, the alarms of 5 nearby Priuses start blaring, providing a fortuitous cop distraction.
In the mythbusters episode I mentioned, they rode alongside the vehicle in another vehicle.
BTW, you need not be totally blind as a gps will tell you where you are, and provide road map info. The day is coming when cars will rive themselves. What is missing is traffic data and the ability to use it.
I’d smash the sucker into a bridge abutment, instead.