Peer-to-Peer Networks and the Fourth Amendment:
Over at the CYB3RCRIM3 blog, lawprof Susan Brenner has an interesting post on law enforcement use of peer-to-peer networks. Peer-to-peer networks are sometimes used to distribute images of child porn0graphy, and law enforcement occasionally go undercover as network users and send search queries for such images. If they locate the images on the suspect's hard drive, they obtain a warrant to enter the suspect's home and seize his computer.

  Susan's post considers whether government access to private hard drives using the peer-to-peer software constitutes a Fourth Amendment "search":
The computers in [these] cases were in homes. Was it, then, a search for the law enforcement officers to access the hard drives on the computers to locate and copy a file or files (which, arguably, is a seizure)?

On the one hand, you could argue it was a search because we have an intrusion – a virtual kind of intrusion – by law enforcement into someone’s home. On the other hand, you can argue this is not a search because [the defendants] both "opened the door" for law enforcement officers to "enter" their computers by installing and using the file-sharing software.

. . . [More broadly,] If I link my computer to a network, have I lost any Fourth Amendment expectation of privacy in the contents of my hard drive?
  I think analogies to virtual space work pretty well here. If a user installs a peer-to-peer program and voluntary opens up a portion of his hard drive to anyone who happens to do the same, then the government is free to search that portion of the hard drive just like anyone else. See Katz v. United States, 389 U.S. 347, 351 (1967) ("What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection.").

  On the other hand, merely connecting a machine to the Internet does not relinquish a reasonable expectation of privacy. Steps that make it possible for someone to break into your computer don't eliminate a reasonable expectation of privacy in your computer any more than having windows and doors eliminates a reasonable expectation of privacy in your home. See id. at 352 (noting that a person who made a phone call from a glass phone booth "did not shed his [Fourth Amendment rights] simply because he made his calls from a place where he might be seen.")

  Of course, there may a need to draw difficult lines here, as what it means to "invite" people in versus "breaking in" can be fuzzy in some cases. But that's true in physical home cases, too, as decisions involving screen doors, open parties, and the like help show.