If this happens I'd suggest everyone at the NAPs deploy some type of MAC-layer
filtering. It's far to easy (and seemingly common) for providers to forget to
set "next-hop-self" on NAP routes .. so what I foresee is providers selling
transit via the exchanges ...and then never seeing the traffic bound to
*their* unsuspecting peers.
'sho ip bgp neigh 192.41.177.x adv' and look at the next-hops.
I know that some providers have been doing it for years, probably
with automated expect-grep-type scripts..