FWIW, I've had success deploying the Spring Social Showcase sample on GAE and it worked with minimal change. (If I recall, I only needed to add an appropriate appengine-web.xml to the project.)
But one reason it worked so well is that the showcase uses an embedded in-memory H2 database. This means that Spring Social never had to deal with persisting to GAE's bigtable, but it also meant that any data stored in the DB would not survive an app restart.
To do this right, we'd need an implementation of UsersConnectionRepository that could persist connections to bigtable. Some have suggested that a JPA-based implementation would be appropriate for this job. While I believe it could be done that way, I also think it might be simpler to have an implementation that natively persists to bigtable.
In either event, the persistence of connections is the one major outstanding issue in making this work in GAE.