Virus coat proteins spontaneously self-assemble into empty shells in aqueous solution under the appropriate physicochemical conditions, driven by an interaction free energy per bond on the order of 2-5 times the thermal energy k(B)T. For this seemingly modest interaction strength, each protein building block nonetheless gains a very large binding free energy, between 10 and 20 k(B)T. Because of this, there is debate about whether the assembly process is reversible or irreversible. Here we discuss capsid polymorphism observed in in vitro experiments from the perspective of nucleation theory and of the thermodynamics of mass action. We specifically consider the potential contribution of a curvature free energy term to the effective interaction potential between the proteins. From these models, we propose experiments that may conclusively reveal whether virus capsid assembly into a mixture of polymorphs is a reversible or an irreversible process.