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???
The GoodGuys had the Alpine DVD version of NAV for I think $2700 including installation, however I don't know if I can trust their skill. I know a place called California Mobile Sound in Brea that will charge $2800 or so for the CD-rom version of the NAV + a set of Clifford Concept 400. Kind of high cost, but their installers are one of the best around here.