One of the comments from the techcrunch post:

there is no hypocrisy here. We have an API that gives users 100% access to all their content on Posterous, including the original media that was uploaded, in it's full size.

This API can be found here: http://posterous.com/api/reading

There is nothing stopping people from writing Posterous exporters that go to other services. In fact, Wordpress released a Posterous importer a few months ago.

We support data portability. It's the user's content, not ours. It's our privilege to host it for our users, and we are working hard to be the best service out there.