ArcWatch has a nice description of what is to come in AGOL subscriptions (a write-up of what I heard at Esri for the most part) and provides these packaging and release details (but no pricing):
The new functionality offered as part of the subscription will be accessible through ArcGIS 10.1 for Desktop, the ArcGIS map viewer, and ArcGIS Explorer Online. Organizations will be able to purchase monthly or annual subscriptions based on the number of users, storage capacity, and page views.
ArcGIS Online subscriptions will be in beta in fall 2011. To learn more, register for a free webinar to be held on December 6, 2011.
(I don't subscribe to ArcWatch, an e-mail newsletter; I'd love to see an RSS feed or Twitter account for it.)