One the points that I left unsaid, however, is that
there may be many, many reasons -- both technically
and business-wise -- why an ISP would want to port-
filter, or for a better generalization, "suppress"
some traffic. For instance, blocking p2p traffic,
or a known worm, whatever. And there very likely
may be busine$$ reasons, as well.
A corrollary: Is it a denial of service to suppress
(dampen) a BGP route when it flaps excessively? Or
perhaps an bloack-hole a RBL entry? Most would say
not, certainly depending on the reason (self- and
Internt-preservation and stability), but certainly
someone could arguably make a federal case out of
it (levity implied) or similar suppresssions or
blocking of traffic.
VoIP brings these issues to the forefront.
In any event, it's going to be interesting to see
how this evolves.