Oddly, Nissan radios apparently are not completely identical from model to model... the fact that certain Nissan/Infiniti 2005 CD changers can play MP3s but the 2005 FX still cannot means I would not hold my breath.
Besides, what are the MP3 player owners who don't have iPods (like me) going to do? You still need a universal solution... the first one to figure this out will be a very rich person.