A federal appeals court has ruled that police can use evidence gathered by a vigilante hacker. The key point, which as I understand it is consistent with a long chain of precedent, is that the hacker in question didn't coordinate his activities with police before doing his work. This seems, though, to open the door to a wink-and-nudge method of getting around the Fourth Amendment. Police announce they need help with such-and-such a case, and swarms of anonymous hackers remove the pesky requirement that a warrant be obtained before a search.
The particular method employed by the hacker in this case—a virus attached to a piece of publicly posted child porn—also raises the notoriously thorny question of the "perfect search."