marc, if article cancellation is made cryptographically secure, there is the possibility that articles can be made uncancellable. of course, if there is some wild card that allows the backbone cabal to cancel articles remotely and after the fact, then i suspect usenet will remain vulnerable to forged cancellation messages. but maybe not.
What you are describing here is an alternate method of cancellation, not a forgery of the main way of cancelling. Of course, if they really want such an alternate method of cancelling, let's write it for them, so that it also uses signatures to check authenticity.
i see your point about backbone admins refusing to traffic in certain kinds of messages, but as a veteran of usenet from before it was even called usenet, i assure you that other admins would quickly fill in the gaps in connectivity. this has happened many times.
All the more reason to allow the backbone admins the power to not pass anonymous articles. It won't work, they'll feel like they're in control, and everyone wins. Eric