Justice Scalia wrote the opinion:
When the knock-and-announce rule does apply, it is not easy to determine precisely what officers must do. How many seconds’ wait are too few?...Justice Kennedy's vote was needed for the majority, and he wrote a separate opinion, denying that "violations of the [knock-and-announce] requirement are trivial or beyond the law’s concern" and that "the continued operation of the exclusionary rule, as settled and defined by our precedents, is not in doubt."
Happily, these issues do not confront us here. From the trial level onward, Michigan has conceded that the entry was a knock-and-announce violation. The issue here is remedy....
Suppression of evidence... has always been our last resort, not our first impulse. The exclusionary rule generates “substantial social costs,” ... which sometimes include setting the guilty free and the dangerous at large...
What the knock-and-announce rule has never protected, however, is one’s interest in preventing the government from seeing or taking evidence described in a warrant. Since the interests that were violated in this case have nothing to do with the seizure of the evidence, the exclusionary rule is inapplicable.
That's not how the dissenters saw it. Justice Breyer worried about letting the "police know that they can ignore the Constitution’s requirements without risking suppression of evidence discovered after an unreasonable entry." For a spirited defense of the exclusionary rule, read the whole thing.
UPDATE: The press is doing a bad job of reporting this case! I keep hearing and reading assertions that the Court said the police didn't commit a violation, when the government conceded that they did! This case was about what remedy was available for the violation.
ANOTHER UPDATE: Nina Totenberg gets it right.