Since I just answered this questions in the forums, I thought I would post the same information here. I expect that there will be JSR 286 support in Spring Portlet MVC as part of Spring 3.0. As for a likely release date, you'll have to bug Juergen for that.

We had been hoping to do the development and testing using the Pluto 2.0 portlet container, since that is slated to be the reference implementation for the spec. However, the work in the Pluto project is going slowly (which is what I think is holding up the spec going final), so we may do testing with some other implementation like OpenPortal, JBoss, and/or eXo. Those projects are all releasing portlet containers that are independent from their entire portal, which I like better for testing purposes.

If you would like to see some of the gory details of how the support is likely to be implemented, Juergen and I have been making plans for this under JIRA issue SPR-4259.