I know this is a little late...
...for your girlfriend's trip. But I was looking for the same thing for the same reasons for an upcoming trip to Japan (Google Maps/GPS functions). Tokyo is one of (if not) the most confusing cities in the world to navigate, and you need as much help as you can get. And "war-walking" for open WiFi APs on an iTouch/iPad is waaaay more difficult than one would think, as most Japanese homes/hotels/businesses either use wired Ethernet or lock their APs.
Sorry, I'm rambling.
I guess it's now possible to rent an iPhone 3GS in Japan for use in Japan through "Global Advanced Communications". If you have a jailbroken iPhone, they also rent SIM cards, 3G high-speed data cards, and pocket cellular WiFi hotspots. Pick up and drop off are available at the airport, and return can be by prepaid mail.
Also, like alohamark mentioned, JCR rents iPhones (4 or 3GS), or Android phones (blyech), but their rates seem really high to me. They rent SIM cards too, but from the way it looks, only for non-jailbroken phones (honestly, their fine print is pretty confusing, so I may be getting that bit wrong). Delivery is available at the airport (fee) or at your hotel.
Hope this info is useful for anyone on their way to Japan.
Cheers,
Mike