I've seen a couple of posts about problems connection to Level3 Networks and/or Cogent. Here's what my fine ISP has to say in the message of the day:
W E L C O M E T O P A N I X
Cogent Connectivity Loss (elr) Thu Oct 6 13:52:51 2005
Some sites on the Internet hosted by Cogent Networks have been unreachable
from Panix since Wednesday morning, due to a spat between Cogent and our
provider, Level3 Networks, who are no longer passing traffic between their
networks.
We are working on getting new connectivity in order to reach these sites,
and hope to resolve the problem by Thursday evening.
-- Ed