It would be nice to have a little link (maybe next to the report link) which returns the posting/PM (datetime, author, subject, message, signature) in xml (or txt) format, signed with a gnupg key created for the forum. PHP has support for GnuPG so it wouldn't be that complex.
That way you could easily archive important postings or PMs and everybody could verify that they haven't been edited in case you need them again later.