OpenAM 13 is scheduled to be released in Q4 2015, according to OpenAM Roadmap.
One of the new features I am looking forward to is Stateless Sessions for Scale and Elasticity. There is a slide (Customer Scale: Stateless Sessions and Managing High-Volume Digital Services) that can better illustrate this feature.
Basically, what we know of OpenAM now is it can only handle stateful sessions. This means in a highly-available deployment, failover storage (OpenDJ used as CTS) is required. This makes deployment slightly more tricky and at times cumbersome. It is especially true if multiple data centers are involved.
In some deployments where session failover is not required, a stateless session architecture is much preferred. Cleaner and faster. SSO product like CA SiteMinder is stateless by default.