In DSpace 1.7.0 RC1, the XMLUI "Page Not Found" page responds with a 200 OK, rather than the necessary 404 Not Found error.
I believe this used to function properly in 1.6.x, but it seems to be broken now.
Obviously, we want this to return a 404 Not Found, in order to ensure that search engines do not index "Page Not Found" responses.
Furthermore, it has been discovered that all other XMLUI Error pages (Cocoon Errors or Invalid Continuation errors) also respond with 200 OK. In addition, it seems this affects all 1.6.x versions of DSpace, as the same responses can be found on http://dspace.mit.edu (currently running 1.6.0 with patches), and http://researchspace.auckland.ac.nz (currently running 1.6.2 with patches).