Mazda did change the lug pattern on the newer gens - now 5
I've heard/seen pics of the '89-'91 Rx-7 spare and they do fit into the trunk of the NC (2006+ miata)
I believe that other mazda new gens run with the same lug pattern/offset - ie. mazda3, mazda5, mazda6, rx-8, but I believe that they all run with different tire sizes - so although the spares from those cars will fit - I'm not sure if the final diameter will be correct for the 2006. I have a mazda5, but I haven't taken the spare out for comparison. If I do, I'll post my findings.