I find it suss that the specs say 18" for the coupe and some dealers are saying they will have 19". Usually the dealers know less than we do about the car and usually just refer to the specs. Who do you believe - some dealer or official specs? I know if I went to my dealer today asking about the wheels they would just refer to the specs.
My advice for 2010 coupe buyers is to make sure 19-INCH wheels are specifically written into the contract if that's what the dealer says is on offer. That way if in a few months you get your Z coupe with 18" wheels your dealer will be in trouble for giving false information and you will get your 19s