Notables features of cognitive radios have made them a technology of choice for wireless communications and motivated the notion of cognitive radio cellular networks in which the network may also operate opportunistically on spectrum white spaces. As spectrum handover can be considered as a new cause of reliability impairment, a reliability framework for cognitive radio cellular networks is proposed in this paper to discuss how cognitive radio features may help compensating the impact of spectrum handovers. For the sake of servicing different classes of traffic, reliability differentiation is also addressed.