I add a couple of provisos to Tim's post below. Justice Kennedy's concurrence makes clear there are not five votes to limit the exclusionary rule in other areas:
Today's decision determines only that in the specific context of the knock-and-announce requirement, a violation is not sufficiently related to the later discovery of evidence to justify suppression. (emphasis added)
That being said, its true that Justice Scalia's reasoning could be extended to other areas of the law if there is another retirement from the Court. Scalia's arguments against exclusion are:
(1) that police discipline and public interest lawsuits are an effective deterrent to violations;
(2) that the costs of its application -- letting the guilty go free on a technicality -- are large;
(3) the violation is causally attenuated when the police could have discovered the evidence if they had complied with the law in a hypothetical counterfactual world.
As Prof. Tracey Maclin's brief for Cato argues, why wouldn't this reasoning also permit introduction of evidence in a case like United States v. Chadwick, where police had probable cause to search a 200-pound footlocker in their possession, but did not obtain a warrant before prying it open and uncovering marijuana? There's no principled line to draw between a case like Chadwick, where police have probable cause and almost certainly could have discovered the evidence if they had complied with the warrant requirement itself, and Hudson, except stare decisis, once you accept Scalia's policy arguments against the exclusionary rule. The implications of the decision for the warrant requirement is surely one of the most troubling aspects of the decision.
There is one ray of hope for the no-knock rule. In his concurrence, Kennedy says that a widespread pattern or practice of abusive entry is "grave cause for concern." Translated from lawyer-ese, this underscores a threat to jurisdictions that systematically violate the no-knock requirement. That threat is class-wide Section 1983 damages under Monell v. Department of Social Services, which makes localities liable for a pattern or practice of police violations of constitutional rights. Were a majority of the Court willing to robustly police systemic knock-and-announce violations against municipalities through the vehicle of class-wide statutory damages, that might well force some systemic reform of police practices in troubled jurisdictions.
Conceivably, as a deterrent matter, this outcome might improve upon applying the exclusionary rule to enforce knock-and-announce violations. (If, after all, Hudson had come out the other way, we might have seen, as Justice Breyer notes, an expansion of "no-knock warrants" -- warrants that excuse the cops, before the fact, from complying with the knock-and-announce requirement based on pre-search judicial findings of exigency.)
Of course, I'm quite skeptical that the Court will follow through on the liability threat. But that's where civil liberties litigators need to turn next.