You are correct, 50 state emissions legalI believe the Nismo will be fine in all states... Cali is questionable but as long as you don't get a nice set of test pipes to go with that I don't think you'll have a problem.
NISMO Cat-Back Exhaust System 350Z / G35
Application: 2003-2004 Coupe, 2004 Roadster
Complete 3-piece, cat-back exhaust system
Stainless steel construction
9 lbs. lighter than stock system
6 HP and 3.5 lb-ft gain to rear wheels
50-state emissions legal
Precise bolt-on installation
Double-walled tip with laser-etched NISMO logo
NOTE:G35 Coupes must use the Exhaust Extension Item number: G35EXTENSION for proper fitment.