Recent events in Boston inspired this blog post by Orin Kerr:
… Assume the police enter a home without consent searching for Dzhokhar Tsarnaev; does the entry violate the Fourth Amendment? The answer depends on whether such home entries are “reasonable” under the Fourth Amendment, which requires a case-by-case balancing of the government’s interest in making the searches and the scope of the privacy invasion. The constitutional question would seem to depend on whether the searches are reasonably limited in scope (such as limited to a specific geographic area), the dangerousness of the suspect (here, very high), and the strength of the government’s case that the suspect may be in the area and cannot be caught another way.
Read more on The Volokh Conspiracy.