Yeah, I think Brad is right -
The 2002 models will likely have a different wiring harness to support the new DVD/NAV option.
There are lots of 3rd party / aftermarket NAV systems you _can_ add to a 2001 IS300.
Now here is a real question:
If you buy a 2002 IS300 _without_ NAV will the dealerships be able to add the factory NAV to _those_ cars, or is it something that has to be ordered from the factory before the car is produced???