In all honesty, I'd have to check. The worker nodes in our architecture will be able to use PostGIS indexes just like regular PostgreSQL instances, and the master node should correctly handle (partition) most PostGIS functions. Still, we'd probably need to implement parallelization for the && operator, which shouldn't be that hard.

That said, I don't want to misguide you here before going over PostGIS' documentation more thoroughly. If you could ping us at engage@citusdata.com, we'll send you a reply once we know for sure.