I severely doubt this line of thought.. but they didn't make any friends with their "switch from X to us" campaign, and it only takes one bruised ego with a lack of ethics to stir up trouble.

That said, if it's like most other DDOS attacks I've seen reported, the target is probably a user of the service with Posterous merely being unlucky enough to host a particular site someone doesn't like. I vaguely recall another platform got DDOSed recently due to some Israel vs Iran type cyberwar.