The fuel tank is steel; I asked the same question back in January when I visited the Yamaha facility here in the NW Atlanta area to see the R3 and R1M. Theye had a pre-production R3 in blue--gas in the tank and a license plate on back and the tires were scrubbed. I didn't get offered a ride, but it had been ridden on the street recently.
The sides are covered with plastic body plates. I plan to paint the black center portion on my hopefully-soon-to-be arriving red model in matching white. The blue has silver in the center.
Yeah the tank underneath is steel. I'm glad they finally did this, Honda has been doing it for awhile, makes it FAR easier to customize the bike and repair crash damage, or hide crash damage I'll be uploading pics of mine all dismantled tonight