I think I have narrowed down our Media Import failure and I believe this is a bug introduced in WP 3.5.2.
Here is the scenerio:
Import content from blog on server1 to blog on server2. Both WP blogs are part of a multisite network setup. Both running Wp3.5.2. Import fails to import any and all media files and attachments.
Export/Import media and posts will work on the same server for subsites of a single network. Media import fails when it is trying to import media from another server with a separate IP and DNS address.
Downgrade to WordPress 3.5.1 and all works as expected. Same server, same plugins, same config files. No issues and full media import.
All servers in my scenario were ubuntu 12.04.
I believe one of the security fixes in 3.5.2 kills media import from a separate server.