R. v. Dineley, 2012 SCC 58, one of the two impaired cases released today, has a helpful passage on retrospectivity:
 There are a number of rules of interpretation that can be helpful in identifying the situations to which new legislation applies. Because of the need for certainty as to the legal consequences that attach to past facts and conduct, courts have long recognized that the cases in which legislation has retrospective effect must be exceptional. More specifically, where legislative provisions affect either vested or substantive rights, retrospectivity has been found to be undesirable. New legislation that affects substantive rights will be presumed to have only prospective effect unless it is possible to discern a clear legislative intent that it is to apply retrospectively (Angus v. Sun Alliance Insurance Co.,  2 S.C.R. 256, at pp. 266-67; Application under s. 83.28 of the Criminal Code (Re), 2004 SCC 42,  2 S.C.R. 248, at para. 57; Wildman v. The Queen,  2 S.C.R. 311, at pp. 331-32). However, new procedural legislation designed to govern only the manner in which rights are asserted or enforced does not affect the substance of those rights. Such legislation is presumed to apply immediately to both pending and future cases (Application under s. 83.28 of the Criminal Code (Re), at paras. 57 and 62; Wildman, at p. 331).