What Karen's Cross shows is that the end systems don't need to know what the technology of the other system is, and that the conversion is done using automation transparently. In Deployment Model terms, here is the diagram for the RED arrow from the top left to the bottom Right. It shows how this system converts a Direct e-mail message into an XDR message delivered over the NwHIN-Exchange.
ONC is trying to mandate that EHR technology support a standards based transport that when made operational by a Meaningful Use Healthcare Provider will be able to communicate with other Meaningful Use Healthcare Providers. What gets in the way is the lack of understanding of Karen's Cross, that one set of EHR technology could choose Direct, while another set of EHR technology chooses IHE-XDR. These two sets of EHR technology can communicate through services that implement Karen's Cross, which is a totally different internet present service. What is more cool is that because of the common metadata model, these can interact with XDS and XCA environments without loss of clinical content or describing metadata. This is a fine stepping stone, but we must keep moving forward to more capable HIE standards and architectures.
I will further expand on HIE and the next logical stepping stones soon, I have links to this and my other observations on Meaningful Use Stage 2