XML creator Tim Bray has proposed a new HTTP error code: 451, "Legally restricted." The idea is to create an unambiguous code that ISPs can return when a user requests a page that has been censored by a court or government. Note the specific number of the error code. Bray thanks Ray Bradbury in the footnotes.
451 Unavailable For Legal Reasons
This status code indicates that the server is subject to legal
restrictions which prevent it servicing the request.
Since such restrictions typically apply to all operators in a legal
jurisdiction, the server in question may or may not be an origin
server. The restrictions typically most directly affect the
operations of ISPs and search engines.
Responses using this status code SHOULD include an explanation, in
the response body, of the details of the legal restriction; which
legal authority is imposing it, and what class of resources it
applies to. For example:
Here's a sample error message:
This request may not be serviced in the Roman Province of
Judea due to Lex3515, the Legem Ne Subversionem Act of AUC755,
which disallows access to resources hosted on servers deemed
to be operated by the Judean Liberation Front.
(via O'Reilly Radar)