I don't see where they stated it was a DDOS attack. Packet loss can occur due to a large number of different issues.
EDIT: They just clarified with the following: The DDoS attackers have returned and evolved their attack around our countermeasures. We expect to be back online ASAP w/ @gigenet antiddos
Gige's DDOS protection is basically a hosted redirect and filtering system that "leverage[s] the cost of DDoS mitigation amongst a large group of businesses, giving you access to the infrastructure that would normally be out of reach financially." http://www.gigenet.com/ddos-protection.html
But if Posterous is hosted on the Rackspace cloud, why isn't there a mechanism to do this through their existing host? After all, Rackspace has an equivalent service (maybe rebranded?) that would seem to offer protection without needing to go to a 3rd party vendor. http://www.rackspace.com/managed_hosting/services/security/d...
So it's interesting that Posterous needed to go around Rackspace to get a solution.
Gigenet / Black Lotus / Prolexic comes in and says -- we have excess bandwidth, we'll take care of it. Then they proxy clean traffic to you. Unfortunately as we are learning, it's also very expensive.