Earlier this year (February or March) I announced to members of my team that finally, we would be able to specify multiple values in subobjects, but then SMW 1.9 got postponed. I guess a release in August is no longer in the pipeline?
For some details, see [1].
[1] http://wikimedia.7.x6.nabble.com/Delaying-SMW-1-9-td5003651.html#a5013057
Roadmap has been updated