A few days after Esri announced support for GeoJSON in AGOL, they updated the above entry as follows:
Note from Esri (Dec 16, 2014): We are considering this feature to be included in the next minor release after ArcGIS 10.3.
So, it appears ArcGIS Server users have something to look forward to after the 10.3 rollout. It would seem to me that, after implementing all of this back-end support, the next logical step would be to add consumption support in the web and (preferably) desktop clients. (Note: That is purely speculation on my part.)
BTW, the language in the note above is still a little ambiguous so now may be a good time to log in and upvote this idea if you haven’t already done so.
It’s great to see my favorite data format making inroads into the vast Esri user community. Perhaps all the tools I use will finally be interoperable in a modern, web-friendly way.