I know that my current employer is setup to reject /32s from
peers as well as not to send them to peers. (Customers too for that
matter save for the ones that have a bgp session to our null router).
Having one's upstream propagate out /32s that they want null routed
because of an attack probably won't scale past the original upstream.
I guess all the tier1s could setup a whole network of special bgp sessions
to each other's blackhole routers (those that implement this method) set
up to implicitly trust each other's announcements, which in turn means
that said network would need to implicitly trust all their peers'
downstreams. Call me a realist, but I just don't see that happening
anytime soon.
Guy Tal