Mnot’s blog:

Today, the IESG approved publication of “An HTTP Status Code to Report Legal Obstacles“. It’ll be an RFC after some work by the RFC Editor and a few more process bits, but effectively you can start using it now.

Tim Bray brought this draft to the HTTP Working Group some time ago, because he (and many others) thought it was important to highlight online censorship; the 403 status code says “Forbidden”, but it doesn’t say “I can’t show you that for legal reasons.” Hence, 451 (which is also a great tip of the hat to Ray Bradbury).