Orin Kerr at the Volokh Conspiracy discusses the utility of personal computer seizures versus network surveillance as a law enforcement tool.
Personal computer searches will maintain their critical importance in computer crime cases for two very practical reasons. First, no matter how much people store information remotely as a general matter, they tend to keep evidence of crime and digital contraband close to home. Second, it is quite difficult for the government to prove a case beyond a reasonable doubt based solely on evidence obtained from a network. You never know who had acccess to the network, or when, or whether the account was hacked or stolen. As a result, nearly every computer crime case ends with a retrieval and search of the suspect's personal computer(s). Finding evidence of the crime on the suspect's personal computer is damning evidence, quite persuasive to a jury. As a result, even if lots of the action happens at the network surveillance level, most investigations still end up with a personal computer search.
This controversy goes to the very heart of the notion of what constitutes a persons computer. Mr. Kerr might be right when he says that most people keep "digital contraband" close to home, but I suspect that for a growing number of people, their information stores are scattered over physical computing resources whose actual location they have no idea of. Consider a person who has one or more web e-mail accounts. That person's computer effectively uses a remote mail server as data storage. Consider this blog. The majority of a person's intellectual property may actually be stored on a remote server owned by the Google corporation.
In the extreme case, consider a person whose operates exclusively from coin-operated Internet machines or Internet cafes to transact his business. It's perfectly possible to do this, provided one is willing to take the risk that passwords may be captured by spyware on the client machine. Where is his computer? But on the other hand, if the network is to be considered a virtual computer, all kinds of Fourth Amendment problems associated with a "vacuum cleaner" approach to surveillance will be encountered.