While Esri discussed the fact that existing ArcGIS for Dekstop users would soon gain access to ArcGIS Online in this year's Q & A, the details were not discussed. Here's how it will work, per Esri Product Business Manager, Rich Turner:
An organizaiton with a customer number will receive a single ArcGIS Online account with the number of named users matching the number of ArcGIS for Desktop licenses. Each named user will receive 100 credits per year. And, this will situation will continue for the organization so long as maintenence is paid. Said another way, the payment of maintenence is the cost of the subscription. ArcGIS for Desktop, as he put it, is just a "premium app" for ArcGIS Online.
Jack Dangermond asked at the plenary if Home Use licenses should also gain access to ArcGIS Online. While the attendees seemed to say, "yes," he didn't make clear the corporate decsision. Per Rich Turner, management agreed that Home Use ($100 non-commercial use licensees in the U.S. for example) will indeed gain access and credits. The only caveat: the desktop software and ArcGIS Online are not for commerical use!
All of this will roll out after the user conference.