This upcoming code-a-thon was news to me, and in a way it’s quite exciting because of the context. HL7 International has run FHIR connectathons at each of its working group meetings since 2011, and these have brought a lot of maturity to the standard, and moved ownership of the draft standard away from the modelling community to the implementer community where it really belongs; Project Argonaut has brought more urgency, using Meaningful Use as a vehicle, and people like Dr. John Halamka and Josh Mandel have been very vocal about its importance.
But this is HHS, which has put a lot of support behind the NIEM Health Domain. To be fair, there could be massive crossover potential between NIEM and HL7, since both are more or less resource-based, using what NIEM calls Exchange Packages, and FHIR calls Bundles. In the long view, it’s not much of a stretch for a health information package (message or document) to contain information formatted as both kinds of resources, bundled as JSON, backed by XSD or CAM.
For anyone interested in Integrated Case Management and crossover potential between Justice and Health, this is definitely exciting. What's probably most important here, though, is the message about SMART on FHIR apps operating with an Argonaut API. This is complete blue sky at this point, but when you look at the number of EHR vendors involved with Argonaut (MEDITECH, Cerner, McKesson, and Epic, to name a few), this could become a reality very quickly.